IMO, the must haves (other than the XPH) are:
Hyperconscious, Malhavoc Press
Untapped Potential, Dreamscarred Press
Apart than those, selecting from among the various options might be trickier. I'd go with more Dreamscarred Press stuff, myself. Well actually I have, personally.

I have yet to get the Mind Divine (their latest work), but it looks very promising. And I won't be using Elan, so yeah, their Elan race book wasn't for me. Other than these two, I can (and do) recommend their PDFs on psionics. It's been refreshing to see something really good come out for a system that frankly should've been better supported at the 'official' level by now.
On that note, I would suggest steering clear of the Complete Psionic offering from Wizards of the Coast. In my opinion, it wasn't much chop.
I've yet to pick up
Mindscapes and
Monsters of the Mind, but will be doing so shortly. Can't comment there though.