Now, look carefully at the paragraph above. Notice the 'order' in which I wrote the sentences.
1) Find a product or service that compliments your book's subject
2) Become an affiliate (if you can) so you make money when you promote it
I didn't say it the other way around.
Don't just look through affiliate programs and pick the first 5 or so you come to. This will take away value from your book because readers, once they take a look at what may be low quality products, will then be upset with you for recommending them.
Pick quality products and services. Try them out first if you can. Email or talk with the creator of the product/service.
This can add tremendous value to your book. Not just because You are the one providing more valuable help to your reader but also because you are generating some extra income as well.
But the value to your reader HAS TO come first.
Please read that again... the Value to your Reader HAS TO come First.
Give value to your reader and they will in turn give value back to you – in more ways than you can imagine.
Ok, so how do you find these valuable products and services to offer your readers?
Go to Google.com and search for your subject matter.
If I was going to do a Pet Name book, I would do a Google.com search for possibly "Pet Supplies."
Notice the links at the top in a grayish background and the links down the right side of the page.
Product owners (or someone) is paying Google.com to display these links. If the person is paying to display the links they've probably got a product/service to promote. And will probably be willing to let you promote it as well.
Just because these links show up – or because they show up at the top – doesn't mean they are 'quality' products/services or the best products/services. It just means someone paid the most to get them there.
Please do your research, just like you did with the content and pick the ones most appropriate and the ones most valuable – to your reader.
Another place you can go is ClickBank.com (http://www.clickbank.com/):
More appropriately, go to the ClickBank Marketplace
There you will notice that the products are listed by subject and you can drill down into each subject and find more specific products (all digital products) that you can be an affiliate of and promote.
In each section the products are listed by popularity so the most popular will be listed first. This doesn't mean it's the best product for you – just that at this point in time it is the most popular.
Research the products carefully before choosing.
Lastly, let me tell you about one more place you can look. It's a site called Commission Junction.
This is a great site to find affiliate products you can promote. You can easily sign up for an account and then search their vast supply of products/services based on the subject of your book.
Between these three powerful resources, I'm sure you'll be able to find 5 or so products/services that are high quality and will provide value to your reader.
How many should you choose? I'd at least provide one per each section or chapter you have. That way for each chapter of information you've given your reader a recommended resource for additional assistance.