Honestly the handbooks I didn't care for. I bought them all, but only really for the maps and the couple of units/weapons in the back. 90% of them I didn't care for.
The originals I only liked the unit rosters, and some of them weren't so complete (steiner).
My favorite books were the 2 volumes of the NAIS atlases. I've read both of those dozens of times, and if it wasn't for those I'm pretty sure I wouldn't have invested tens of thousands in the game since. Unfortunately, they weren't that popular, and the new management tries to avoid detailing the universe like that (and aren't very military oriented, which may be a bit of a reason too) so the chances of seeing those again are nil.
TROs sell, so we see LOTS of them. Rulebooks sell, so we've seen a bunch of those too. The old scenario books apparently didn't sell well (I thought the starter books were EXTREMELY well done) so they are pretty much discontinued as well.
It all comes down to popularity. Unfortunately the BT fan base isn't large enough that anything but the highest popularity books see repeats. Perhaps if we could get a LOT more fans, the less popular books could still justify the costs to produce.
I really wish they'd do a "we'll make it if you pre-order enough of them" sorta system. Kinda like kickstarter. Lets say I want a book on some big age of war battle on 1 individual planet that was some obscure reference. The book would be 100 pages, have X count of various pics, and require X amount of time to produce. They start a KS and if they get to 1000 pre-orders, they'll write it. Stupid folks like me could buy 10, other folks maybe half, but if they get enough the book gets printed. I would think that'd work, though I'm no line developer.
But hopefully we will see it soon, so they can wash their hands of it and work on the future products (which I hope are 2500-3025, as all the new rules/system are slowing the game down immensely).