Proof reading, even more so than editing, is all about obsessive attention to detail! Has every word been spelled correctly? Is the grammar correct? Have the appropriate typographic conventions been observed? This is the time to root out any rogue apostrophes, to spot missing quotation marks, to find there where it should be their, or compliment where complement would be correct.
Because proof reading occurs after the book has been typeset, this is not the time to be making major revisions. Your typesetter may charge you extra if you request too many changes. For this reason, it’s important to choose a meticulous editor who can prepare an accurate copy for typesetting.