What do I buy when I'm confronted by shelves and shelves of books? Here are my guiding principles:
Poetry collectionsThese show up all the time at quality used book sales and discount booksellers. I've purchased enough so that I can put a poetry book in every student's hand when I teach poetry. This is so helpful! We can talk about characteristics of poems--"Some have short lines, and some have long lines" or "Some poems have formal rhyme schemes, while others do not"--and kids can page through their books and find examples right there! Then they trade books around and look for more.
I keep my poetry books in a separate tub and only have them circulate occasionally, with a few book displays during the year. This prolongs their lives and keeps them in high demand!