Specifically, you want to look at this page:
http://savage99.com/models.htm
Unfortunately, that page is not nearly complete, nor does it factor in all of the model letter weirdness that Savage introduced over the years.
You've got to remember that Savage worked in two distinct time periods for these rifles -- up to 1920, and post 1920, and the model letters were reused at various times throughout the gun's history with different characteristics.
Guns marked 1899 were made prior to 1920 or so, those marked 99 are post 1920. There are some transitional guns that can be very tough to identify.
Even worse, though, Savage reused model letters from the post 1920 guns for other post 1920 guns with different features.
Oh, and did I mention that Savage didn't visibly mark model letters on guns until very late in production? Some models had model letters stamped on hidden areas, but it's not always a given.
It can be very confusing.
So, if you find yourself a nice Savage 99 (especially an older one) you need to know a couple of things to pin down the model:
1. Serial number.
2. Chambering.
3. Stock characteristics (shape, checkering, no checkering, shape of forearm, etc.)
By far the best resource on the Savage 99 is Doug Murray's book, which you can buy from Amazon (although it looks like its out of print)
http://www.amazon.com/exec/obidos/ASIN/B000UTHTQY/savage99
Your best bet is a gun show.