Spine poetry, for lack of a definition, is a creative way to use the books around you to create a poem. To create a poem, you “simply” stack your books with their titles facing out and work top to bottom. I say “simply” because it actually takes a lot of thought and playing around, plus you’re limited by the books that are at your disposal. In other words, if I could walk into the library I would either be overwhelmed by the options or I would be able to find titles that matched what I was trying to say.
Spine poetry falls into a category of poetry known as found poetry. According to Poets.org, this type of poetry takes existing texts and refashion them, reorder them, and present them as poems. In other words, found poetry is the literary version of a photo collage.
The point of Spine poetry is to create art without the intimidation of a blank page. Perhaps you end up using a title to create a deeper poem. Perhaps you become inspired and starting using more than the title of a book (ex a entire quote!). Here are some of the poems that I’ve created using my books:

Pawn of Prophecy (David Eddings)
Swallow me whole (Nate Powell)

Stardust (Neil Gaiman)
A game of thrones (George R. R. Martin)
And then there were none (Agatha Christie)

Lost ocean (Johanna Basford)
I regret only everything (D. Mars Yuvarajan)