NASA has released a a couple of e-books on the X-15 suborbital rocketplane.
X-15: Extending the Frontiers of Flight by Dennis R. Jenkins is a very complete (644-page) official history of the program. It was published in December, 2010. The paperback sells for $35 on Amazon but you can download it for free here.
X-15 Research Results (NASA SP-60) by Wendell H. Stillwell is an older technical report. It’s from 1965, so an original hardcopy is probably hard to find. You can download that one here.
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