The problem with SEO books, and even online SEO documents, is that they are written by people with a limited view of half of the equation. Most SEO people do not understand search engine development or how a multitude of search algorithms work together. The good ones are limited to testing things and then, if these approaches work, using them or writing about them. However all it takes is for one or two modifications in a search engine's algorithm (or an equally limited search engine employee with a poor understanding of the web) to invalidate years of SEO. With SEO there is one very important rule even for dealing with SEO techniques even from acknowledged SEO experts: Verify before using. But don't just rely on one source or even on books published prior to the last major search engine algorithm update.