Innes & Glass "The Illustrated Encyclopedia of Cacti" is a really good and beautifully photographed book, although some of the nomenclature is a bit dated and there are some surprising omissions (the Trich section is a bit ordinary, for example). Subik & Kunte "The Complete Encyclopedia of Cacti" is also a good book and includes some really good background info on each plant. If you're at all interested in collecting plants from a certain region, this book is a great reference as it groups plants by location. Again, there is a bit of omission here and there, but overall it's an excellent book. Preston-Mafham's "Cacti the Illustrated Dictionary" is probably one of the better guides; however this one is limited to purely the small globular and miniature species. It does have its limitations, but is does these types really, really well.