NASA published a comprehensive (if a bit dry at times) ebook, X-15: Extending the Frontiers of Flight [1] that covers the development and test program. If you're ever near Dayton, Ohio, the National Museum of the USAF has X-15A-2 [2] and an XLR99 [3] on display.

[1] http://www.nasa.gov/connect/ebooks/aero_x15_detail.html

[2] http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id...

[3] http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id...