If you're familiar with markdown and your book is basically text with some images, I'd strongly recommend Pandoc Markdown.
Pandoc Markdown-as-input is probably the best-supported input format for Pandoc, as far as "reasonable defaults for outputs in other formats" is concerned, and it's broadly compatible with the norms of other markdown styles.
You can always drop into latex, include custom CSS headers, etc. It's also a format where the "formatting" won't generally get in the way of actually-writing, unlike HTML or LaTeX (speaking on behalf of mere-mortals, here).
Pandoc Markdown-as-input is probably the best-supported input format for Pandoc, as far as "reasonable defaults for outputs in other formats" is concerned, and it's broadly compatible with the norms of other markdown styles.
You can always drop into latex, include custom CSS headers, etc. It's also a format where the "formatting" won't generally get in the way of actually-writing, unlike HTML or LaTeX (speaking on behalf of mere-mortals, here).