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.