Want to know the difference between Nolan and Noland? Ghiberti and Ghirlandaio? Otto Dix and Gerhard Richter? Help is at hand, in the fulsome but very reasonably priced shape of The Art Book (Phaidon pounds 19.99): alphabetical entries, with a colour illustration of a typical work, the briefest of notes and cross-references. Any well-endowed bookshelf should probably have one.