Ok, so I'm just starting to get involved with affiliate marketing and ppc. Can someone explain to me why a keyword might show up in the Google Keyword Tool as having 6,000 monthly searches, but when entered into the SEO Book keyword tool, it has 0 searches across all search engines? Am I missing something?
The Wordtracker results (which is what seobook's tool uses) are quite inaccurate. They get their data from Metacrawler and Dogpile, but those sites have a very small search market share. Google meanwhile has about 3/4 of the market and their stats include their search network. For most beginners, the Google keyword tool is sufficient. For better results, you would have to invest in paid keyword research software. For now don't worry about exact numbers, think more in terms of relativity to other keywords. You'll never get 100% of the search volume for any keyword anyway.
google get most of the traffic thats why its result always greator than other sites so stick with that.