I've got a thing for popular reference books. I can control it, though. I never let it get out of hand - only three or four books a night, and never enough to get more than a little bit educated.
The ultimate reference resource is wikipedia, of course, and I love getting lost on that website clicking from link to link, but its ubiquity had made the good old-fashioned coffee table reference book seem outmoded. This is a great shame, because physical reference books allow something no website can emulate - the flick-through. You can go to a random page on wikipedia if you want. But what you can't do is flick through all its entries rapidly and stop when something catches your eye. You can have serendipity, in other words, but not bounded serendipity.
Bounded serendipity is the function of a physical flick-through-able book. You pick it up. You flick through. You see something that intrigues you. You stop to make use of it. You repeat the exercise. It isn't random but nor is it structured. It provides value through being semi-random while making use of your own eye as final arbiter.
Bounded serendipity is one of the main reasons why physical bestiaries are important. Flicking through a bestiary will not give you purely random monsters like a generator could, but it will result in certain entries leaping out at you as you flick - influenced not by a dice roll but by subconscious creative forces. You need a monster for stocking a hex lair and as you page through your Monster Manual the Peryton stands out to you as somehow appropriate. It does not stand out through fluke alone but due to a mixture of fluke and deep creative tides churning somewhere in the ocean behind your eyes. It's a heady mixture of luck and unconscious choice.
The bounded serendipity of the flick-through also works well with spell lists and treasure tables for similar reasons, although a special mention has to be made of books of maps - whether fantasy maps or the road atlas variety.