Reddit reviews Private Pilot Test Prep 2016: Study & Prepare: Pass your test and know what is essential to become a safe, competent pilot — from the most trusted source in aviation training (Test Prep series)

Basically just search for whatever is confusing you but here are some of my favorites:
https://www.youtube.com/user/garrywing
https://www.youtube.com/user/MzeroAFlightTraining
https://www.youtube.com/user/undaerocast
https://www.youtube.com/user/FlightChops
https://www.youtube.com/user/KLAviationVideo
https://www.youtube.com/user/kingschoolsvideos

With these links there is no need to purchase anything besides an ASA book for memorizing the stuff that you don't quite understand yet. The written is all about wrote memorization. You'll tie it all together as you progress with your training.

EDIT: The book with all of the possible questions, the answers and brief explanations: http://amzn.com/161954234X

Make sure you have enough saved to complete the training to CFII without stopping for long periods of time in between finance lows.

Just getting your commercial severely worsens your chances of getting a job out of flight school.

I've seen it happen, but the chance are few and far between.

In terms of reading material get yourself a Test Prep Book and start memorizing the correct answers. The knowledge test is silly and your score on it doesn't affect your career at all as long as you pass. Best bet is to memorize the answers until you can pass the test. You won't be missing out on information or not learning stuff, your instructor will ensure you know what you need to know for the checkride. Also as /u/Cropgun mentioned, the Rotorcraft Flying Handbook is a good start. If you get it, don't read into it too seriously, just get yourself familiar with the terms, again, your instructor will teach you what you need to know.