How to Spell Like a Champ is a particularly useful book, and for broader a field than most people might think. Written by three important staff members of the Scripps National Spelling Bee (2 word panelists and the director), it contains hundreds of useful tips that you can use not only in spelling bees, but in your everyday writing, studying, and school work. It has mnemonic devices and several strategies for remembering rules and roots. If you'd like to improve your spelling or learn how to study (like a champ!), this book is most likely a good one to read. The context is understandable for nearly all ages (depending on maturity level), and very concise in giving information. Some bonuses are: advice on staying calm, puzzles and games around the end, a CD included, as well as some history on the bee (and Webster, dictionaries, etc.) that you can read up on. Illustrations, photographs, and features are also in "How to Spell Like a Champ"
Reading the book is a simple approach to achieving a bigger vocabulary and improving your skills in spelling (and spelling words you don't know). I'd say that it would be compatible with most readers willing to give it a shot.