I have chosen an alternative question today as I had no real answer to the default question and I didn’t want to do that two days running.
It is also relevant to the regulars here on the blog. We are all heavily leaning towards publishing, have published, are waiting to publish or want to publish.
I have two answers to this. First up I like books or more rightly the companions that make you think that the content should have been core right from the start. Books that solve so many problems or things you have had to wing during a session and finally you have an answer for. Players will always try to push the limits and one of the first things they will always want to do is swing two swords not one, pull two pistols not one or fire some machine gun from the hip. They do it in the movies so PCs should be able to do it in game. Books that make those problems go away are the special ones.
Answer two is more about book and page layout. Some books just make using them so incredibly easy. I don’t like the game system FATE but for book design I have seen very few to beat it. I have put an example below. The chapter tab on the side of the page makes thumbing through to find the right chapter easy, the cross references are obvious and and not buried deep in the text. The page contains no art but remains attractive to the eye by breaking up large blocks of text. I have held this book up as an example of great page layout and it does make the book a joy to reference even if the game is dreadful (IMHO).