I recommend Michael Stackpole's Warrior" series as a starter, though I expect the only source is Stackpole's own website now. It's a great introduction to the Battletech Universe as conceived by the original designers of the tabletop game. You get to 'see' many types of mechs in action--their strengths and weaknesses--while following the political events of the era.
I've read a fair number of the books, and you have to watch out for the timelines, because the Clans didn't come into the scene for quite a while.
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