I used the SCCM Comprehensive Critical Care: Adult text for the bulk of my reading throughout the fellowship year. I read the guideline from various and sundry societies.
I used a bunch of miscellaneous sources for digging up some minutiae - like specific side effects of specific antibiotics, etc.
I wish I could tell you that made me well prepared, but I felt the test was full of intricate detail that I either knew or didn't. I ended up clicking enough correct bubbles that they issued a certification in my name, but I'm not sure how I passed. I don't feel it was a test whose content assessed my knowledge as a specialty-trained physician.
All that doom and gloom aside, the pass rate for the last few years has been mid-to-upper 80% of test takers, so the stats are on your side!
Good luck with it!