I took the Extra without studying at all and I got 35/50. Of course I have a lot of experience and probably benefited from a guess or two as well. But it is like the VEs told me, 35 is all we ever needed and how we got there just doesn't matter. It is a good thing we don't have to sign with our test scores in the EXTRA SUBBANDS!
I expect some day the Extra Class will be eliminated. The Extra privileges bear absolutely no relationship to the test materials, it is but a bragging right. At one time you had to be an Extra to operate a space station (which explains the amsat related questions in the test), and maybe repeaters way back when, I'm just not sure. But these days the only spiff you get is something like 500 KHz on HF to operate the same modes at the same power levels you can do as a General, and a somewhat distinct callsign (and that's going to be a 2X2 in most places).
For study guides, I used to like the old ARRL License Manuals. But that was a long time ago, when tests were still given at the FCC office and there was no published question pool. Using that, I was able to ace the old Tech/General written test back in '82. I don't know how it is these days.