This happens to me, as well, and only ever since the .8* versions. Previously, it worked fine.
It seems to crash at different times. Sometimes, it crashes as soon as Calibre starts up. Other times, I'm able to browse through my library and poke around alright. This morning, I was even able to play with my preferences for a while, making some tweaks and smoothing things out (the crashing started prior to this, however).
I'm unsure how to get a log or debug info for this. When I tried to restart into debug mode earlier, it crashed xserver.
> calibre-debug -c "import sys; print sys.version_info"
sys.version_info(major=2, minor=7, micro=1, releaselevel='final', serial=0)