What content is there in the BMM that is not available in the core rules books? I have been playing since the 90s and prefer combined arms games, making a mech only book seem rather questionable for me. What was added that is not available elsewhere? ???
The book was intended to pull existing content together into a better form, rather than introduce new content. As such, if you just want new stuff, the book probably isn't for you.
At the same time, there really was a lot of work put into that new form, so if there was anything about your ability to understand or use the TW rules at the table that frustrated you (or you just don't feel like having to haul around two very big books everywhere, plus the very large errata documents for each, plus IntOps if you're playing in the Dark Age), I think you'll really appreciate the BMM. For example, I'd hate to have to figure out any aspect of displacement using TW alone, or hidden units.
In terms of pure new content, there are:
1) the Battlefield Support rules. These are beautiful, in that they give you in five pages what the existing TW/TO rules take literally 70 to provide: artillery, air support, and minefields. They do the base job, but leave the focus solidly on the mechs actually on the table.
2) the quirks section. Old quirks often received heavy rewrites to make them less broken, to cover more situations, to add errata. Nine all-new quirks were added. Then all this was applied to a new list covering every base mech chassis ever made, which had never been done before. Quirks are still optional, however: this just makes it much better if you do like using them.
3) the "simplified" rules variants. Dropping mechs is there (from StratOps), but in a version that doesn't require you to use the aero maps or to know anything about the aerospace rules. Fire and smoke is there, in a modified form that doesn't take all day to calculate fire and smoke spread. Some TO terrains are there, but without all the annoying bog down stuff.
4) the Common Misconceptions section. Semi-new, in that we've never really done anything like this before but ultimately it's a restatement of the existing rules. Still, I've heard again and again from players all over the world that "man, I read that and I found things I've been doing wrong for decades".
One small note: there's no new fiction in the book, because other than some cute one-line blurbs for the weapon section there's no fiction or fluff in the book whatsoever. This was a deliberate decision, to keep the focus on the rules.
I wrote up a blog post a while back that explains the thinking behind the book in detail. It should help you give a better idea as to what it is and if you might find it useful:
https://bg.battletech.com/news/news-and-announcements/what-is-the-battlemech-manual/It's out of print at the moment, but there's plenty of copies in secondary channels still. However, the pdf can be had for 10 bucks, and will be upgraded to the new printing once that arrives in a few months, so that might be a good, affordable way to get a feel for it.