why would you pay for textbooks when you can get all of them for free online? heres the current working link to annas archive. download whatever books you want or need, plus a shit ton of scientific articles. every time ive looked for a book on there ive found it, and often multiple duplicates, even for extremely obscure control theory textbooks that only a few thousand people in the whole world would care to read. the scientific articles are less complete but ive had good luck there too.