If you don't know what hexadecimal, octal, etc. is, then that shows that you probably don't know a lot about Computer Organization, in which case, I would recommend getting Patterson & Hennessy's Computer Organization.

For hashing and algorithms, any book or course about algorithms would probably do.

Salting and such are more related to database security and the security of distributed applications. Any book or course on computer security should explain these.

If you don't understand what ASCII and other encodings are, check out: https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/

Design Patterns by the Gang of Four... but it hasn't really aged well. There are a bunch of online resources that list both design patterns and more importantly anti-patterns. Example: https://sourcemaking.com/design_patterns