Google & Other Search Engine Advertising Info

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I'm in the process of changing my advertising around to include more advertising on the internet. I've talked to some companies about advertising on Google and other places but I have no clue what I'm doing. Does anyone know of any good books or sites that might be of help in this area or does anyone here work in that field. Its not cheep so I'm trying to learn what I can before making a decision. The companies I have talked with have the same sales info so they are not allot of help.