Coaches Torbjorn Karlsen and Ruff Patterson compiled an excellent, straightforward guide to cross-country ski training in "How To, When To, Why To: A Norwegian Model Training Guide & Programs for Cross-Country Skiers."
The Third Edition of their book provides example training taken from successful elite athletes, discusses general physiology and training theory and then provides general and specific guidelines to help construct training plans for all ages of skiers. This book is The One book to have for ski training.