If you're making iPhone apps to try to make money, then marketing is key or only your friends will ever download your app. When most people hear the word 'marketing' they think of telling people about a product after its been built, but I see marketing as much more than that, and in fact I think the most important type of marketing is the basic understanding of the market in the first place, and that comes before you make you app. Why? The most important factor in getting people to buy your app is having an app that people will want! If you can do that the rest is much easier. You have to think about out what kinds of apps people want to buy, why they choose one app over another, how many people want something like this, what you can do to make your apps desirable. Once you've got a handle on all that, then you actually make your app and go ahead with the advertising and PR part of marketing to get the word out. So here are some books to help...
The Business of iPhone App Development by Dave Wooldrige and Michael Schneider is an excellent on book that covers market research, app design & design for your promotional materials, different pricing and revenue models for apps, working with the press, connecting with customers, and running promotions. There's a lot of hands on techniques and information in this book you can really use. (I was the tech reviewer for the first edition of this book, Dave just updated it in 2011 to a 2nd edition.)
In App Savvy Ken Yarmosh takes the reader through a masterclass in app product strategy, managing app development, marketing and engaging with customers. The book is packed with sophisticated approaches that successful app publishers are using, but all the material is presented in a accessible easy to read form. Ken's expertise comes from publishing his own iOS apps and working as a consultant on more, but he goes further and includes interviews with plenty of other successful app developers, marketers, and industry experts. App Savvy can give you an up to date understanding of the app store marketplace, will help you understand where your own app plans fit in, and its filled with guidance towards success. I especially love App Savvy because Ken introduces the ideas of Customer Development / the Lean Startup, which have been very influential on my own apps business. Whether you are an indie app maker, or working in a larger company, I cannot recommend this book strongly enough.
You probably want to get some press for your app, but can you afford a good PR firm? No? Yeah me too. If you are going to pitch the press yourself you MUST read 'Pitch Perfect' from TUAW.com bloggers Erica Sadun and Steve Sande. They tell you what its like on the inside to get hundreds of app pitches a week, and they coach you on how to get them to notice your app.
Another good iPhone specific marketing book is Jeffry Hughes' 'iPhone & iPad Apps Marketing, Secrets to Selling Your iPhone and iPad Apps', and it covers similar ground to Wooldridges book, and had lots of good examples of how app publishers promote their apps, and some financial models to help you make sensible plans.
Suzanne Ginsburg's 'Designing the iPhone User Experience' has some excellent advice about how to perform a competitive analysis of the market for your app, and lots of detailed information about user research and prototype app testing - key 'before you build it' marketing.
'Positioning, The Battle for Your Mind' by Al Ries and Jack Trout was first published in 1981. Its been perhaps the most important marketing book I've read, it really changed how I think about how customers feel about the products they buy, and was tremendously helpful in understanding the vast gap that exists between how you feel about and see the app you are making, vs how all those iPhone users our there might feel about and see your app. Really, it comes down to the fact that pretty much no-one will feel anything about most apps because for most apps, no-one will ever even know they exist! Positioning is a huge challenge for companies with millions to spend on marketing, so how can this book be relevant to an independent iPhone developer with a micro budget? The lesson I took away was to make something that people can imagine what it is... that they could recognize what the app will do... that they can make that jump to wanting to check it out when they first see the app icon, name, or a screen shot when they are in the app store or perhaps a website. Check it out, its a great read.
Priceless: The Myth of Fair Value is a wonderful read about behavioral economics. It should make clear to you why in app purchases can be so profitable, and why you need to sell multiple things in order to get your freemium game to pay. You know what - real customers are _not_ doing ROI calculations in their head when they consider buying your app!
Also very influential to how I think about my iPhone business are the ideas of Customer Development and Lean Startups from Steve Blank and Eric Ries. Customer Development is about taking the risk out of startup businesses by figuring out what to make and how to sell it before investing lots of money on sales. Another way of looking at the basic idea is to say its about figuring out what people want to spend money on that you are capable of making, and it lays out techniques you can use to get there. Steve Blank explains this in his book 'The Four Steps to the Epiphany'. You can also check out the more recent The Lean Startup: How Today's Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses. If you get a chance to hear Steve or Eric Ries speak, take it.
Starting your own business as an iPhone app publisher means you've got to deal with a bunch of legal and admin stuff. Whether you hire a lawyer or not, I recommend learning some of this stuff for yourself, how else can you be in-charge of your own business? For those of us in the USA, Nolo is here to help. A few of their excellent books to check out include 'Legal Guide to Web & Software Development', 'The Small Business Start-Up Kit', and 'Consultant & Independent Contractor Agreements'.
I'll leave you with one final book recommendation which can serve you well in life generally, and especially well if you take on some iPhone app consulting / contracting work! 'Bargaining for Advantage' by G Richard Shell is a great and easy read that will explain what master negotiators do, will help you defend against getting bullied in negotiations, and will help give you the confidence to find agreements that work for you - including decent contracting terms and rates.
More book recommendations: iOS Programming, App Design, Games & Unity.