...I voted Field Manuals and Plot Sourcebooks. If there was a third, I'd vote 'novels' - I haven't been impressed with many of them, they seem so - plastic, unreal, with stiff, unrealistic characters - but you'd have to find a really good writer. Frankly, I think novels might be the most important, but that isn't what the poll is for.
The TROs are wearing thin - I've always described the game as 'chess with a couple hundred different types of pieces", but the choices are getting overwhelming. Its getting to the point where I don't use them because I already have a mini/RS for a fire upport/brawler/fast recon/whatever-unit or I don't have the time to look up their new/experimental/non-tournament tech. Its getting to the point I have to choose ERA, then units. Frankly, I think you should wipe some of them out by just decreeing them as extinct or isolated custom units.
FM's may not sell well, but how about FM: U? Why not use it as a guide? For a long time now, the FMs have been my favorite books. And plot sourcebooks - we're in this pit of a plot device called the 'dark ages', ever since Wizkids. Advance the plot, and get us out of it into something new (sounding), besides another Succession War or something.