Chris,
A lot of people have suggested an Amazon affiliate situation, and unless my assumptions are wrong it just won't pay. The following is a copy/paste from a quick spreadsheet assuming some basic information. Amazon gives affiliates 4% of a standard sale, I assumed a CTR of 1% (which I think is very generous), I guessed a 2% conversion rate, and an average sale of $45.
You can see that your monthly take will still be quite low.
Traffic 1300000
CTR 1%
Delivered 13000
Conversion rate 2%
Converted 260
Avg Sale $45
Your cut/sale $1.8
Your take $468
It is entirely possible that your CTR would be higher, but I tend to think not.
You said that with Adsense you only earned $400 in a month, which means you were getting a $0.30 CPM. This is pretty low, and even in this ad market I think you should be able to pull off $1.50 or more. An Ad network would help you do that, and your traffic is high enough that they would be willing to talk to you. Of course this means ads on the site.
This is what I would do:
* Add a forum; with your traffic I think you could have a community discussing issues pretty quickly. I would probably do this as a sister site given your niche name. Of course this 'only' increases your pageviews.
* Optimize the site for ads, forget Adsense, and work with the ad networks. Better still, try to beat down doors of people who might want to advertise directly.
* Despite the small payoff I do like the idea of selling books related to the topic and would try it. I tend to like the idea of dealing directly with a specific publisher however; your cut will likely be better.
* I like the idea of premium content as well. I despise sites that show you only part of a solution, but your site shows that you have enough taste to do this in a way that users would embrace.
Good luck,
Jer