well man...just use your brain for keyword research. there is no tool that is reliable for your work.Google ad words is for paid campaign keyword research and we just get a rough idea about keywords and remaining other tools i don't have much idea about them.
For a discussion comparing keyword tools themselves, there's a current discussion that's worth looking at.... [webmasterworld.com...]
In Google Adwords Keyword Tool, It showed the Monthly search of around 3,68,000
In Seobook Keyword Tool - It showed the Monthly search for Google as 61,380.
What were your settings on the Google Tool? Either broad match (rather than exact match) or global monthly volume (worldwide) rather than local monthly volume (ie, within country) could account for the difference.
Not sure where the Seobook Tool gets its data. It may be from Wordtracker, which extrapolates data and tries to project monthly volumes from a small initial sample size. IMO, Google is currently giving the best data.
Google have the biggest data set, but it's geared towards advertisers, and includes things like ad impressions, which skews the results rather heavily.
You can only obtain really accurate data with real-world stats, e.g. via Adwords advertising or web analytics for a site that already ranks. That said, the numbers are usually in proportion so if you pick the highest volume relevant keyword you're not likely to be far off.
It really depends on what degree of accuracy you're comfortable with.
I think you're going at it the wrong way.. You need to be seeing which types of keywords are right for your site, through research on performance related keywords. A simple way of doing this is using Ad Words and exporting related searches to your Ad Groups. This way, you're sourcing and filtering keywords based on your own criteria. Just pull a few generics off the top of your head related to your industry to get started.