I must admit that I have an issue with wanting to buy books, especially books on herbs and health.  Lately I've cut back significantly,  and frequently revisit those I've already aquired. 

This is more of a practical list..... the skeleton of my collection.  I would like to think that I have a favorite out of all of these books, but I really don't.  All of these are different in what they have to say or address, or to whom they are wanting to speak.  Most can be found at amazon, but I would encourage you to support smaller stores such as the Bulk Herb Store when you purchase.  BHS offers media mail shipping for books only purchases.

 Practical Herbalism by Fritchey.  Concise and complete info on herbal actions (the medical part) with a specific breakdown on 47 common herbs.  Offers formula suggestions on specific ailments.  Very detailed info in this book.

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Staying Healthy with Nutrition by Elson Haas, MD.  Incredibly complete volume on everything you could ever want to know on nutrition.  All aspects of foods, including carbs, fats, proteins, vitamins, minerals and accessory nutrients are discussed.  Steps to building a healthy diet along with specific food analyses are also present.

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The Herbal Medicine-Maker's Handbook by Green doesn't give specifics on herbs, but goes into the actual methods of preparing them for use in detail.  Though I'm much more of a visual and need examples kind of learner, this book does a good job at making tinctures, salves and teas less daunting that first time.

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For beginning herbalists I would recommend  The How-To Herbal by Keith.  The How-To Herbal is more of a general guide with individual herbs discussed and formulas given, as well as how to use your formulas. 

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I also love using field guides so that I am positive in my identification when collecting.  The two I use the most are Peterson's field guide to Medicinal Plants and Herbs and The New Age Herbalist by Mahey and McIntyre.