As Shaykh Uthaymeen advised, in this book - The book of Knowledge - a student of knowledge should take to buy books when can afford and built library. I always thought of it this way. It is very beneficial that one buys beneficial books when one has money to buy them and when books are found before a time when one does not have money to buy books and when books are not found. Many books come out and then goes out of stock and print.
So insha-Allah Buy BOoks When you can afford and make your library.
Insha-Allah you will always benefit from them.