The Classic Battletech Website (
http://classicbattletech.com/index.php) has a ton of free information.
The timeline covered goes well beyond where we are playing, so all information past 3023 is subject to (and most likely will) be different in my Battletech Universe.
I recommend the House Sourcebooks which can be found in the Downloads Section (
http://classicbattletech.com/index.php?action=downloads) for, in my opinion, the best 'fluff' of any RPG ever put on the market.