I've been a serious Nordic skier for about 20 years, getting out every possible weekend in the winter; serious, but sort of a permanent "upper intermediate". Along the way, I've collected many cross-country ski books and guides, from the 1960s to the present.
Gillette and Dostal's has remained my favorite, and overall the most useful. It was issued by the Mountaineers press, and is better than the Mountaineers later effort. A good practical guide for the novice through intermediate, and if you have only one, this is the one to have.