Here’s a paraphrase from memory of an instruction sheet that came with the main Swedish encyclopaedia back in the 90s. I treat all new books this way to keep their spines from cracking. And they just can’t have enough of me.
1. Put book on table, spine down. Fold down left cover, smoothen inner edge, fold down right cover, smoothen inner edge.
2. Fold down 15-20 pages to the left, flatten firmly with finger along inner edge.
3. Fold down 15-20 pages to the right, flatten firmly with finger along inner edge.
4. Repeat steps 2-3 until the book is spread out flat in front of you and open at the middle.
5. If it’s a hardback: close the book and knock the front edge, opposite from the spine, firmly against the table a few times.