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Good question. I suppose it depends on what you use the book for most and how current you need it to be. As you've probably noticed, the general interface is not significantly different between Mavericks and Yosemite, but you will notice differences here and there that may become frustrating if you're using a book on a previous version of the operating system.
Of course, if you just need a general guide to the main functionality of OS X, the Mavericks book might be enough. You'll just need to be aware of differences when the book and your screen don't match up.
And, obviously, the Mavericks book doesn't cover any new features introduced in Yosemite, which are many. If you want to know about those, it will indeed be worth getting the Yosemite edition.
I hope this helps. If you're not convinced yet that you need the new book, perhaps just use the old one for now and wait to see whether you're missing key features or frustrated by the differences. The Yosemite edition will always be around for purchase if you find yourself needing it.
Senior Editor, O'Reilly Media