I think there are more crucial things to think about than God. He is immutable, and the immutable is irrelevant. When instead of liberation it is used to control, it becomes actively hostile to human nature.
As for lending books, I loaned someone The Cyberiad, Penguin edition and purchased a new one which was from the last century and is probably worth more than I paid for it, so that's good. I also loaned Notes from Underground and it was not returned. Both times I stressed that I wanted them returned.
I also do not want to sour relations with people because of lost literature, however I do like to re-read texts for better understanding. I think that if in a few months he doesn't voluntarily return the book, I will just buy it again.
Why don't you re-purchase lost literature? If there is anything that I spend copious amounts of money on, it is books.
Also; if someone is pressing you really hard for you to loan them something, asking them for collateral isn't unheard of. One method is to swap books, where you take one book you really liked and they take a book they really liked and swap it and then discuss it. I haven't personally done this, but it seems reasonable to demand something of value for value of your own.
It's sad how friendships are overshadowed at once by material imbalances.