If you find a good one, please repost. I'm also on the elusive quest for solid jumping legs. I think the title of the book I'm looking for is, "take 20 years off your life and then start riding", but I don't think anyone sells that one!
I have been unable to find the book as it pertains to horses, so for now I'm using pilates DVDs to help build up my own muscles. Ooh, maybe that book Pilates for the Dressage Rider. It's not jumping, but it does pertain to muscles needed for riding.
You just have to see your distance...you don't have to like it.