Here are some ways to gather books for your classroom library
Hold a book drive
Send home a letter with your students announcing a classroom book drive. Be sure to specify what kind of books you need otherwise you may get adult books and toddler books as well. You might hold a competition between classes (for secondary) and offer a pizza party to the class who donates the most books. Be sure to check every book you receive to make sure it is appropriate for the classroom. When a book meets approval, write on the front inside cover "Donated By:" and the student's name. Some teachers like to make up a bookplate on the computer and print off several so that all they have to do is write in the student's name. Use thick clear tape and tape the bookplate on the front inside cover.
Use Scholastic and Troll Book Clubs
You get points for every book ordered in these book clubs. Collect the points and use them to get free books for your classroom.
Lots of people sell books at garage sales for under $1.00. I once got 2 boxes full of great books for about $10.00. Once again, be sure you look over the book carefully to make sure it is appropriate for your students.
Ask teachers who are retiring or leaving the profession
Some teachers who leave do not want to take anything with them. Ask if they want to take their books with them and if not, see if they will either give them to you or sell them to you for a good price.