You can actually read them for free online: https://ccrma.stanford.edu/%7Ejos/Welcome.html
I picked up printed copies on Amazon regardless, both because I wanted to support the author after reading almost a full book online, and because I sometimes prefer reading a physical textbook (the "spatiality" somehow helps me remember better).
I'm a Software Engineer without any background in signal processing (or audio), and stumbled upon his books when an FPGA hobby project of mine involved a simple audio pipeline for which I wanted to do some sample rate conversion. Since then DSP has become a hobby on its own.
I actually read the "Digital Filters" book (the second in the series) first, before reading the first book about DFT fundamentals, and it worked pretty well.
Same here really, a software engineer by trade with a growing interest in audio. I’ve been really impressed with the quality of these books so far. Very meticulous attention to detail with _very_ few errors compared with most other books of its kind.
The problem is that my equipment always reports things in "real" time and frequency.
Having to make this jump always confuses students horribly.
Look at all the grief that Monty got about why not 24/192 downloads. He actually had to create a video about it:
How long does it take to work through the textbooks ?
I see JOS' other texts in the repo, are you aiming to complete those too?