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I currently use Adwords and Overture. I get the best conversion (as best as I can learn) from Adsense on content sites in my target field.
I use Findwhat and Ah-Ha, but the click/day are very low on those. They don't deliver much, but don't cost much.
Are there other networks that I should look at? I am particularly interested in content sites in my field, but I don't have the time to contact individual webmasters (who probably don't have time to talk to me).
Also, these content sites might need editorial. Write a press release on the e-books and e-mail it out. You might get some free publicity, plus pick up PR & traffic from any URL published.
How about advertising on engineering directories related to your niche?