I purchased a version of the MERM that was at least 15 years old in my preparation through Amazon and used it during the test too. It was my primary reference and along with the NCEES practice test. While some gaps were uncovered while using the NCEES test during practice, I used Wikipedia and some old textbooks to brush up on some of more esoteric topics.
While I don't want to extrapolate my individual experience to say that this is okay or advisable, the manual itself was extremely valuable during the test and preparation. This old edition probably has 80% to 90% of the content of the most current text and I purchased it for ~$20 along with its companion practice test book. This route certainly provided more "bang for the buck" than brand new books.