RPG books usually have terrible graphic design--which is sad, because they are one of the very few types of books that actually need good graphic design.
I do not mean "good graphic design" in the boring, anal-retentive sense of "do whatever you can to make the book look expensive" (WOTC has that covered, since they have money, and most other game companies have it covered to the best of their ability) or in the commercial sense of "good graphic design makes the content look more interesting than it actually is" (RPG companies are all over that, they know all about that), I mean in the sense of "organizing the presentation of the entire corpus of information to be processed repeatedly at high speed by players and GMs". (Information design + graphic design, really, as DerikB points out in the comments.)
Good graphic design in this practical, engineering sense: Novels don't need it. Magazines don't need it. Encyclopedias could use it but don't need it. You can take your time with these--if you weren't taking your time you probably wouldn't be reading these things in the first place. On the other hand: Instruction manuals need it. RPG books--up there with maps and tourist books--do desperately need it. And we haven't even gotten started on modules--adventure modules need it more than anything ever.