How old is too old for a book?

[PTPoeny]

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So, I am an MD/PhD student and several of my friends from my original class gave me their clinical books when they graduated. A few of the books haven't put out an updated edition since, but most have. A few other books they bought used and are as old as 2001 editions.

While I really like saving money, I don't want to compromise what I am learning either. Does anyone have any advice about the age of pocket reference books vs. question books vs. review type books? Does it vary by subject/rotation at all?