In reply to Michael Gordon:
And does it depend how you use the book?
A big flat book just makes your leg thicker (probably fine).
A small thick book just above the knee then narrowing quickly gives an edge and homething to hook with (possibly not OK?). Does a hardback give you 'the edge'. (pun fully intended).
Maybe we are about to enter a whole new era of different shaped knee pads. You could get packs of different shaped inserts to go under your pad. You could sell route specific shaped pads. I'm predicting 150+ comments on the ethics of knee pad modification. Go for it!