You can get them for free at the library. No need to spend professional development money on them.
Most material is available for free on the internet, but like most open source material, you have to compile it yourself and that takes time. Good books save you the time by including important dependencies, normalizing notation, etc.
I've been in software for 20 years and have not yet felt like I needed to read anything that wasn't pretty mainstream.
I imagine one's professional reading is more academic when working on things like hardware or compilers, but most of us (for better or worse) are just working on CRUD apps in a marketing-adjacent space or maintaining a corporate Java monolith.
I don’t know what the CRUD equivalent would be, if there is one, but at the time random projections and large deviations were fringe math, and only the basic texts were available everywhere.
The same goes for recent books on applied machine learning or recent tools or libraries intended for professional practitioners. I think libraries are reluctant to buy books they think will go out of date quickly.
It's not easy to even ask for a book. I've asked librarians for books before, and I even have a friend currently working in a fairly large library request a book for me (with evidence from me of interest in it), and the request was refused.
The real books worth reading are those that make you a greater expert than the majority of the population. Libraries don't benefit much from storing books that will only be read by one or two people.
Additionally, if you have an alma mater, try looking with them. Most universities will give full library access to alumni if you are part of the alumni association, including journal articles, newpaper subscriptions, magazines, etc. Dues aren't too expensive and lifetime membership is reasonable, at least for me.
I'm currently living near Purdue University and that's definitely the situation here.
But I'm from Puerto Rico, live in the city, graduated from the University of Puerto Rico and that's not the situation there. I can, probably, at best, get them to give me access to an old computer of theirs in the floor and access journals through there.
It just depends on the area.
But it does exist in some areas.
Nowadays, you can get a digital copy instantly instead of waiting for another location to send the book to your library.