I'm sure people will chime in and re state my below list, but here I go.

Here are the things that I ask myself as an affiliate.

**** Does this website come off as trustful?
1). Will my website's clients, be interested in this product?
2). What will my commissions be?
3). What is the minimum commission required for payment?
4). What is the payment schedule?
5). What payment methods are offered, do they offer paypal?

As for ebooks/real books, I would let the end customer choose what product they want to purchase.