If you can only afford to buy one rune book I recommend Taking Up the Runes by Diana Paxson. Not only because the author includes a wide variety of valuable information and suggestions, but because she extensively quotes and references many of the other most often recommended rune authors including Aswynn and Gundarsson. Because of this through buying this book you get the advantage of the knowledge contained in the other authors' books as well. This is a wonderful advantage for people on limitied budgets who can't afford to buy all of the indivudal books on their own. Although ultimately I do think getting a variety of different rune books is the best way to go, when that isn't an option, especially starting out, this is th eperfect book for you.
I liked the set up of the book very much, finding it both easy to use and easy to break down into small segments for an effective rune study program. Each chapter featuring two paired runes and including the Icelandic, Norwegian, and Anglo-Saxon rune poems, as well as suggestions for divinatory meanings and magical uses. Reading the original rune poems is wonderfully enlightening and allows the individual to get a feel for what the runes might mean on their own, without the modern filter of current authors' interpretations. I found it very useful to go over the rune poems for each rune and then mediatate on them for a little while and form my own opinions before reading further and seeing what the modern ideas about each rune was. As I said before Paxson includes not only her own ideas but also the highlights of the interpreations for each rune by the other major rune authors which provides a very well-rounded view of the meanings. While the ritual and meditation suggestions might not be to everyone's taste they do serve to illustrate the possibilities, and could easily be tailored to suit the individual. This is not a historical a study of the ancient runes, but rather a modern exploration of the uses of the runes in the world today and it serves that purpose very well. The magical applications are often aimed at very modern needs like car travel, and should be points of interest even to those who don't intend to use the runes magically.
The book was designed based on a series of classes taught by the author and lends itself very well to study groups, particularly those structured (as the author suggests) over a longer period of time. Ideally I would suggest reading it in small segments, either one, or possibly two of the paired runes, at a time so that you could get the most out of each section. Personally I have read it through, used it for a study group, and keep it on hand as a convenient reference. It is generally the first book on runes that I recommend to anyone and the first I go to to check anything.