Affiliate guides are for sale all over the web. If you know someone in the business, ask them for a recommendation before spending money on one. Look for testimonials from customers, then visit their web sites and see if you like what they offer. If the web sites don't impress you, think twice before buying any affiliate guides they recommend.
You can also go about getting recommendations by finding affiliate web sites that seem to be successful and then writing to the owners. Asking the owners how they got started and inquiring about any useful books or guides they reccomend can help you gain valuable insight and guidance from someone who knows. You may be surprised about how many people are happy to offer advice.