The linked pdf contains notes explaining HFT circa ~2011. The About section explains it's sourced from the author's blog, and was also used as lecture notes for an undergrad course. I've not read the whole thing, but at a minimum, the brainteasers contain some good nerd-sniping content.

For those wanting more recent additions, the author seems to have more updated thoughts here[1], as linked on the LinkedIn profile to which their former blog redirects.

[1] https://blog.headlandstech.com/2017/08/03/quantitative-tradi...