Thank you to The Man in Seat 61! He’s generously provided detailed information regarding train travel in Spain and step-by-step instructions for booking your rail passes. He walks through everything you could possibly need to know–the what, when, where, why, and how of Spain Railways. He breaks down the fares, the classes, and the trains themselves (including interior photographs of the various trains). A 10-second comparison helped me to decide on the luxurious AVE train!
P.S. He gives great advice regarding printing your tickets:
Print your tickets: When you’ve booked and paid, the tickets should automatically pop up in PDF format in a new window or tab. You can print them out on your own PC printer, and save them to your PC. Personally, I not only print a spare set of tickets to place elsewhere in my luggage, I save the tickets to a dropbox.com account I can access from anywhere.