Users running OS X Mountain Lion version 10.8.5 are beginning to see an “Open in iBooks” button for purchased books in the iTunes Store, replacing the previous “Download” button text. The button, while functionless on Mountain Lion, is yet another indication of the imminent launch of OS X Mavericks late this month.¬†Clicking the button on a computer running Mavericks will indeed open the selected book in the all-new iBooks application.

This isn’t the first indication we’ve seen of Apple’s launch preparations for Mavericks, and it’s certainly not unprecedented. Last year, we saw a mention of iBooks 3.0 leak right before Apple’s fall 2012 media event, also in the iTunes Store. Additionally, Apple began OS X Mavericks training for support staff at the beginning of the month, and developers are now being asked to submit their OS X Mavericks compatible applications ahead of the launch.

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The same button as it appears in OS X Mountain Lion 10.8.4 or earlier.

Although Apple hasn’t officially released the software yet, development of Mavericks has, for the most part wrapped up, with development shifting toward the next version of OS X codenamed “Syrah.”