Do you think the lifespan of the topic or the delivery platform impacted your returns? I don't buy a lot of books anymore but used to drop 40-50 bucks on an O'Reilly book that would be used for years. My LP purchases probably top out under $10 and are for more immediate, short-lived topics (which seems to be what the platform targets with it's digital & updated content focus)
The problem is that it's relatively easy to hire a marketing partner (does PR, etc.). But it will probably cost quite a bit more than your book advance. And most publishers these days aren't setting up book signings, sending out review copies, etc.
When I get pings to see if I want to review a book it's usually from a PR agency.
as a developer-who-can-market i often wonder if itd be worth my time taking a 50% cut to be a marketing partner heheh. not sure i can do it repeatedly for people i dont already know.
Wrote a book about Redux (https://leanpub.com/redux-book) brought about $15K for 4 months of work. (Granted, did no marketing at all)
It always feels strange to say "I wrote a book" and it never seemed to really impress tech people or cause an inflow of consulting work.
But it is very fun and you get to really get into the tech (other projects, sources, blogs, testing ideas).
Prob would do again one day