A Guide to Quilling Flowers by Helen Walter is quite popular, but I have mixed feelings about it. I like the designs in this book, but the book itself isn't that good, in my opinion. It contains wonderful quilling flowers, including detailed explanation of making them: what color and length of strips to use, which quilling shapes to use, and how to arrange them. You can find a lot of interesting ideas there. For example, I used a cactus from this book for my dad's birthday card. The designs depict plants in a very accurate way and succeed to grasp their distinctive features very well.
On the other hand, there are some things about this book I didn't like. First, the pictures showing basics of the technique are so small, that it makes impossible to understand anything, which renders almost useless the whole section (although short, but important). Then, all the designs in the book are photographed "flat", so that you just can't see the details well. I think that with quilling, it's important to view it at an angle. Finally, the implementation is far from being neat: some pieces fall off or are missing at all :( Yellow glue stains on some of the compositions don't look beautiful, either.
All in all, if I'd had an opportunity to check it in a store, I wouldn't have bought this book.