In James Martell's Affiliate Handbook, he mentions that he focuses on optimizing almost exclusively for Google, since they get the bulk of search engine traffic. But there's been a little bit of controversy lately about some of James's sites being penalized by Google. My personal suspicion is that his sites were excessively cross-linked, and some of them might have had duplicated content. But one of the things I have noticed is that the Martell sites are doing very well in Yahoo.
A good friend of mine is an expert at SEO. He explained to me a few days ago that Yahoo's current algorithm is very similar to Google's old algorithm, before Google started trying really hard to thwart SEO's and affiliate marketers. This would explain why Martell's sites are still doing very well in Yahoo. And while Google might still get a lot of searches, Yahoo is no slouch. If Yahoo gets half the searches done on it that Google does, then it's still sending a terrific amount of traffic. (I heard someone say that Google might put the lobster on the table, but Yahoo can still put the steak on the plate.)
Yahoo is more sensitive to on-page optimization than Google. One of the only flaws (and it's a minor flaw) with James's materials is his focus on keyword density and on-page optimization. Off page factors matter much more than on page factors right now, and that situation won't change anytime soon, as far as I'm concerned. It's just so much easier to manipulate your on-page factors than it is to manipulate your offpage factors. Yahoo's a big fan of having your keyword in the URL, but Yahoo's results are heavily related to your backlink structure too.
I recommend James Martell's book and his methods, but I encourage everyone to do some independent thinking too. I disagree with a cookie-cutter approach to webmastering, and to business as a whole. Pay attention to what's going on, and experiment with different things. Learn what's working for you, both in Yahoo, and in Google, and don't be afraid to try something different now and then.
The author runs several affiliate websites, and you can read more about his exploits at his Affiliate Marketing Programs Blog and see one of his sites at Major Millions Jackpot.