I usually give books a try by borrowing from library online archives. If I find myself repeatedly returning to a book either to re-read completely or re-read my favorite lines, I'll purchase. The only books I tend to buy right off the bat are classics or other books that I know I'll want to keep for years. I'm an active reader who really enjoys annotating so if there's a book I know I want to fully immerse myself in or if it's a book that I know will require more of my energy and concentration than is typical, I'll buy so I can annotate freely.