As game players we all think we can design games, but to make a great game you really need to step outside your own enjoyment of playing games, think about what makes games fun and more, how games generate an emotional experience for they player. 'A Theory of Fun' by Ralf Koster is an easy to read 221 page masterpiece exploring what games are and why they are fun. Every 2nd page is a cartoon that illuminates the concept on the proceeding page. This is no gimmick, it really makes the book more accessible and will aid your understanding. Koster gets right to the heart of what the human mind gets from game playing, how people vary in how they like to play, whats happening when players cheat. This book is so good, I need to go read it again for a third time instead of describing it more. Making a game? Buy this book. Scott Rogers' 'Level Up, The Guide To Great Video Game Design' is my second recommendation for game makers. Its a much bigger book that Koster's and instead of looking at the heart of what gaming is in the player's mind, the book takes a hands on look at the elements of computer games: characters, camera, controls, UI, cevels, combat, game mechanics, audio, etc. Its also a very readable book with lots of cartoons, and it will help you understand what you are building in terms of the conventions of all the games that came before, from which your players learned how to play computer games. Its an excellent resource to help you understand some of your choices when you get stuck with some part of your game's design, and it will help you round out parts of your game you've neglected.
Finally on general game design, I'd like to give a shout out to Nicole Lazzaro. I don't think she's written a book yet, but go hear her speak if you can, and get her papers on the 4 Keys to Fun at her website XeoDesign.com.
Typically games means animated 2D or 3D graphics, and you'd be a fool to write a game from scratch without using a game engine. There are a bunch of excellent game engines on the iOS platform, but undoubtedly the two leaders are Unity 3D and Cocos2D (with Box2D or Chipmunk Physics). After writing our first iPhone game from scratch in C and Open GL ES, we've now chosen Unity 3D for our next games, and I have to tell you so far its amazing, brilliant, excellent value for money, and totally fun to use! Despite the name, Unity 3D is good for 2D games too (check out Sprite Manager 2). It speaks volumes about Unity that there are several books now available to help you learn to harness it...
Unity Game Development Essentials by Will Goldstone, and Unity 3D Game Development by Example are two good beginners books, feel free to select either one or both to get started in creating Unity scenes, scripting objects, and making basic games. Unity Game Development Essentials introduces everything by working through the building of a simple first person perspective game where you walk around an island and interact with the 3D world. The example game the book takes you through can't be run on the iPhone though, as it makes use of the terrain modeling features of Unity which are not supported on iOS. Unity 3D Game Development by Example on the other hand takes you through building a handful of casual games without using any features that aren't supported in iOS. The book is aimed not just at Unity beginners, but at beginner programmers too, and it a very easy to follow. If you are an experienced programmer you might find it a bit annoying, and in fact the authors very casual style ends up detracting from the book a little because the humorous section titles just make it hard to look up Unity stuff you need help with.
'Creating 3D Game Art for the iPhone with Unity, Featuring modo and Blender Pipelines' by Wes McDermott is a fantastic book to help you get to grips with Unity. From the title you might think this book is for game artists about 3D modeling, but really the title could have been 'Everything technical you need to know about making 3D games with Unity for iPhone except programming'. It will teach you how to use 3D models, animation, and textures in Unity whether you are creating them yourself, using stock, or hiring a 3D artist. Also, despite the fact that modo and Blender are in the title, this book is for you whether you use those applications or not. The book explains the power and limitations of Unity on the iPhone and has great advice on how to get good graphical performance. It teaches how to use all the different settings for importing graphics into Unity so they will work well on iPhone, and explains some performant techniques for constructing the game world. The author takes you through all this by building up an animated 3D over the shoulder perspective shooter game.