A wise manager once told me when it comes to projects, "Cost, Schedule, Function... Pick any two." He meant that it's possible to bound two of those items, but the third will always be unbounded and won't be within the limits that your interested in. I think that role-playing game books or manuals have a similar pattern but different three items. These are Readable, Presentation and Usable.
Every book is readable if it's in a language you understand, right? Well, that depends on your point of view. I personally have trouble finishing a book if it reads like a college Calculus book. If the book is boring or difficult to understand, it is hard to finish that book. Some examples that I have had difficulty reading are Diaspora and Burning Wheel. With Diaspora, it is just difficult following the writer's train of thought. It could really use an editor going through it with 100 red pens. As for Burning Wheel, the text is just boring. Now to be fair, Luke Crain, the author states that you don't want to read it cover to cover. An example of a game that I love the readability of is Swashbucklers of the Seven Skies.
Most game books need to be usable during play at some point, so usability is a key component. Some take it to an extreme, for example, Dungeons and Dragons 4th Edition; the game's books are really meant to be referenced at the table. Usability is critical part of the function of those books. One of the games that I find difficult to use is Song of Fire and Ice(SIFRP) from Green Ronin. A specific example in that book is armor. The cost is in one section and the stats are in another. This really bugs me and makes it not fun for me to use.
Presentation has to do with the whole package. Is the book pretty? I have been known to buy an RPG book simply cause it's beautiful even if I never plan on reading it. A number of examples are Aces and Eights and Dark Heresy. Both games I own with no intentions of playing unless some else runs it.
There are books that do only one of these well but are still fun game. Take SIFRP, in my opinion, it is very pretty presentation but has very low usability and readability, I still enjoy the game a friend is running for me.
Another example is Burning Wheel, it concentrated on usability but is a boring read and doesn't have that much in presentation. Granted, it's only 25 bucks for the core system, so it's still a bargain.
Diaspora has been getting a lot of love out in the Internet. This is what got me thinking of this post. It fails in all three categories. Some might argue that it has charts at the end of the book which increases it's usability, but if you need to go through the text, you're going to be at it for a while. I really enjoyed the ideas in the book, but it's really hard to get to them. I kept hearing a large number of FATE fans say this is an awesome book, so I ended up finishing it, but it took me a month and some sections I had to reread.
What games do you enjoy reading? Do you buy games just cause attractive layout and art? How important is usability to you?