Probably the most interesting Elan book that I own (and I do have shelf-fulls) is the "Definitive Description of How to Build and Maintain a Legendary Car" in other words Brian Buckland's Lotus Elan Addendum Workshop Manual.It is very unlikely that this will ever be re-printed but it is still available via Club Lotus UK at 50 GBP, which is 20 GBP less than I paid when it came out. I'm actually thinking about getting another copy to sell on in ten years time.
The best Lotus book of all, imho, is "Jim Clark Life at Lotus" by Peter Darley. Peter was the official Team Lotus photographer and his work is seriously good. I have actually bought a second copy as an investment (still in its packaging) this book is already hard to find and will be a sure-fire investment.
Two non-Lotus car books which have escalated in value enormously recently are "The Certain Sound" by John Wyer (GT40 story) and "Lucky All my Life" by Harry Weslake. I think that I've missed the boat with those two.