Chasseur smacked the answer to your question right out of the park with his response! I would second all of the books he recommends- I've read all but two of them and I purged a lot of useful information out of them!

If I would offer some suggestions:

Regarding boxlock/sidelocks. Sidelocks are by far and away the prettiest DRs you can get, a probably stronger. With the sideplates fully engraved, they are just as much a work of art as they are a working rifle. But to me, boxlocks are works of art too, and for a fraction of the price (traditionally) than a sidelock.

I'm just your average working slob without a seven figure trust fund so a boxlock was the right choice for me. BUT, if money were no object, and dropping SERIOUS money on a sidelock would not put a dent into my cash accounts, sure, I would get a sidelock. But in my opinion, the boxlock does the same job as a sidelock.

Also, be sure to handle as many DIFFERENT double rifles in your price range. Each one is different, with a different feel and balance. It's easy to see one and say "I'm getting it," later to compare it to a similar one that fits you better. Double rifles are not like buying a Remington 700 off the shelf; you will be dropping big dollars for one so don't rush into the purchase without first doing what you are doing now.

This forum is a great medium for meeting other individuals that share a similar love of DRs. If you are seriously considering purchasing one, I would assume that most people here would take a few hours out of their day to let you handle and shoot their rifle...there are some GREAT people on this forum.

Before I bought my Merkel 140 in .500 NE I shot and handled about five NE calibers from different manufactures. For the price, feel and pure enjoyment, the Merkel was the best choice for me.

Just my two cents...