It's definitely possible to make a decent amount of revenue from a reference site like an encyclopedia.
Your focus with a reference site should be doing the initial SEO (getting incoming links from authority sites). I have built a similar site before which generated about $10K per month in ad revenue (almost entirely from Google Adsense).
Here is the course of action I would recommend:
- The site should be easy to use and fast to load. That's key with reference sites.
- After you launch, reach out to the library section of college sites. They'll be more than happy to link to your site if it provides the material in a user friendly manner.
- Don't have ads in the beginning when you launch. It'll be much easier to get edu backlinks from your outreach campaign if it's not commercial. Plus it won't bring you much revenue from the small initial traffic anyway.
- Reference sites (if the SEO is done right) can get a lot of traffic. Within a year's time, your goal should be to hit at least 100K uniques a month.
- Once you reach a sizable traffic, then think about monetization. Google Adsense is a safe bet. Due to the niche, the Adsense ads won't pay much in CPC, but you'll make it up in volume.
- Your most profitable Adsense unit be one that starts right before the content. I'd suggest starting with that, and testing other units over time. Don't go overboard with many AdSense units on a single page (that one above content unit will still account for a majority of the clicks).
- Collect email addresses from the beginning. A weekly newsletter about the top 10 topics is a good starting point.
- Once you've exhausted your link building campaign to colleges, go after local high school sites. Those backlinks are also easy to get.