I think works really well for textbooks, with well-placed footnotes, sufficient margin space to make notes, etc.

There are, of course, many other styles, if you do not wish to utilize page area more efficiently.

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If you need nice equations (or even just Greek alphabet), and a no-hassle reference linking system, I can't think of anything other than TeX derivatives that will do a good job.

PS: If OP wants more useful suggestions, we need to know the topic and/or what kind of content the book will contain -- text, equations, floats/figures, photographs, etc.

If I was going to compare them to commerical software, ConTexT is closer to InDesign and LaTeX is closer to Word.

With a graphical tool, one is limited to the automation which the developers are willing to build into the tool and sentenced to handling manually everything else, every time that there's a change, e.g., if you have a keyword block on your opening article pages aligned against the outside gutter and the layout program can't place it automatically and contextually, then every time the article changes from opening to a left to a right or vice-versa one has to make that change manually.

I wrote up a longer comparison once upon a time --- Scribus isn't that much different from InDesign and Quark, so the criticism holds:

While I'm no TeX wizard, I prefer it because it allows one to off-load some of the tedium and repetitiveness to the computer, as opposed to repeatedly solving variations of the same problems by hand time after time after time.

So,

* using Quark is like being chained to a an oar which is covered w/ splinters and mostly broken at the other end and which will randomly break due to being poorly carved (Quark has crashed on me 183 times this year) leaving one adrift or run aground, or sometimes returning the vessel to its starting point (a few of those crashes have resulted in unrecoverable document corruption --- my autobackup folder may contain 2 or 3 GBs of files for a given iteration of a particular project each month) --- the oar can be smoothed somewhat and reinforced (by purchasing or finding XTensions, using XTags &c.) and periodically one is required to purchase a new oar (sometimes just after the previous one has been customized adequately). For some tasks, one can impress any graphic designer as a galley slave to ease the effort for others, but while charts are available, there are no automagic navigation options and every journey must be manually piloted.

* using InDesign is pretty much the same except the oar is smoother and stronger (it's crashed 29 times on me thus far this year), there aren't as many customization options and it's not quite as easy to find a candidate for impressment (though soon it'll be as easy as for Quark). Charts are available, but again, piloting is strictly manual.

* using Plain TeX one has to craft the vessel's oar oneself (as well as the rest of the vessel unless one is typesetting a clone of The TeXbook), but it's as sturdy and as nice a one as one's skills allow and can even be an engine which moves the vessel in and of itself --- it can be difficult or impossible to find people suitable to help w/ either carving the oar or using it though, but once a given journey is worked out, the oar becomes magical and rows for itself except for when one runs into an unplanned for obstacle (the navigation charts are old ones and not often up-dated, with a lot of “terra incognita”), allowing one an auto-pilot option for certain journeys, dependent upon one's skill. using ePlain, an oar is provided, can be customized, and can be enchanted and the charts are okay, but have a lot of “terra incognita” on them.

* using LaTeX, an oar is provided and there're lots of nifty customizations and improvements already available, and one can impress additional oars from CTAN, however on a semi-random basis, adding one oar will break other oars, sometimes leaving one adrift or run aground. One can enchant a set of oars to accomplish a given journey, easing the piloting requirement, and the navigation charts are decent and obstacles are fairly well-known.

* using ConTeXt, a very nice oar is provided, which has lots of customization options, but the navigational charts aren't easily read by a traditionally trained navigator at first, although they are fairly compleat and most journey can be carefully worked out, but once one is, it is quite automatic and there's a good auto-pilot option.

If you are going to make things in Indesign, it should be as a last step process, after the full draft has been made. Otherwise you will slow down your writing by a lot.

Quark is rather outdated IMO, little has changed over the years. It has terrible UI and native keybind controls

I would use some flavor of markdown + some notetaking tool that supports it. A good example would be gitbooks.io, there's other ones out there too.

Latex & Variants is what has always been used for textbooks. Can't go wrong with it

You can write your text in Markdown and convert it to TeX, DocBook or directly to PDF using Pandoc (https://pandoc.org). In my opinion Markdown is much easier to type and Pandoc gives you everything you'll need for styling your document.

I usually start a writing project in pandoc Markdown, until at some point I have many equations, figures, or tables that I have to switch to LaTeX to continue. I know pandoc can support all of these elements but it does not give you as much freedom as LaTeX. Equation and theorem numbering, figure and subfigure referencing, fine tuning the layout of a table ... things like these that are natural in LaTeX require extra effort to be done well in Markdown. And of course, there are things that LaTeX does not do well naturally, for example, code highlighting. Should always pick the right tool for the right task.

Check a tutorial for the basic workflow and you are all set.

Many people have used it and still use it for their books, theses and such. I wrote my thesis with it back in the days. The best part was to be able to just focus on the writing, not formatting.

There is a layout mimicing the layout used by Edward Tufte in his books. That layout is awesomely readable (a highly subjective observation, I know).

I see this 'slogan' frequently when it comes to Tex-based editors. (I realize you are talking about a GUI interface that only uses Tex as its backend... comment isnt really directed to you.)

Is the slogan really true? Perhaps I'm just easily distracted, but I find the exact opposite is true. I spend 1% of the time drafting, and 99% of the time fiddling with Tex, loading packages to make a cool kind of ligature, then ending up reading about the history of ligatures, then writing my own package to create a ligature more consistent with those used in 17th century upper Bavaria, then adding more features to the package to account for monospaced fonts, etc... and finally, at around 4:00 am, I realize I've only written a single sentence.

Maybe it's just me... but the slogan seems like something an alcoholic in denial would say.

In your case, it seems you are not really writing the text... You are making cosmetic adjustments. And it is fine but it should not be done up front, IMO. As for ligatures, TeX should handle ligatures automagically, but I took that as a general point and yeah, I agree -- one can always find things to tweak.

Process-wise, it is better to "just write". Most of the cosmetics is taken care of by LyX with minimal effort. Add a comment for something to check later. Make the process iterative, if you try to write each line as production quality from scratch, I doubt you will ever finish.

For pure composition, you are certainly correct... do not write in the Tex editor... Just compose with WORD or something, and then take care of the formatting in Tex at the end.

(Regrettably, though, I am involved in several projects where the composition necessarily has to be in the LaTeX editor as the format and cosmetic adjustments are very important.)

It seems a lot of the helpful questions and answers on the various LaTeX websites come from some PhD student working on his dissertation. I kind of smile with sympathy at their earnest questions about how to print an equation upside-down and backwards in a table cell while using multicolumn (just a silly example...) I know whoever was writing the question was, like me, surfing the internet and focusing on formatting instead of getting back to work and finishing the damn dissertation.