From my personal experience, Amazon's ploy of suggesting related books has been very effective on me. I've often gone there with the intention of spending perhaps USD 40 on a specific book and ended up spending thrice that amount (happily!).

Hence a suggestion, FWIW: in addition to improving conversion rate, it might be also worthwhile to try to sell more stuff to existing customers. You know what they want and you know they're not averse to spending money online.