In 1998, search engines added link analysis to their bag of tricks. As a result, content spam and cloaking alone could no longer fool the link analysis engines and garner spammers unjustifiably high rankings. Spammers and SEOs adapted by learning how link analysis works. The SEO community has always been active – its members, then and now, hold conferences, write papers and books, host weblogs, and sell their secrets. The most famous and informative SEO papers were written by Chris Ridings, PageRank Explained: Everything you’ve always wanted to know about PageRank and PageRank uncovered. These papers offer practical strategies for hoarding PageRank and avoiding such undesirable things as PageRank leak. Search engines consider unethical SEOs to be adversaries, some web analysts call them an essential part of the web food chain, because they drive innovation and research and development.
Langville & Meyer, Google’s PageRank and Beyond: The Science of Search Engine Rankings, p. 44.