Date is very important in the 500's especially in the Space area. The books should not be older than 5 years. First when weeding get rid of the books that are in bad condition. Keep a list of subjects that you cancel for a source of replacement. As for use, I delete books that do not support the curriculum besides the high interest books for the students.
Fiction is more subjective.
I delete for condition and use. I try to keep up with the new books by favorite author. One good source to help select is to use Award books list for good book. I rather recommend better written books. I hope this helps.