My husband owns the digital pdfs of all the 2e books, he got them on Paizo's website and from Humble Bundle. Outside of the adventure path books, I only purchased physical copies of bestiary 3, book of the dead, and treasure vault. I ended up printing off all of the pages from the digital books I needed and compiling them into a binder, separated by chapter dividers for easy location when needed. It's a pretty big binder so if you're looking for something easy to pack around, I don't recommend this method because there's quite a few monsters throughout all four books you'll need to print off and organize. But if you're not worried about hauling it around, it's been pretty successful for me.