one of the absolute best books about the why's and how's of jumping (esp if you're working at an h/j barn!) is George Morris's "Hunter Seat Equitation". He's mr. H/j himself. You can get a lot out of his book.
I too agree with anything written by George Morris! He's the Guru of Hunter/Jumpers, and there's a reason why he is who he is.
There are also great Video's online of his clinics that are extrememly benefitial. I watch them often and pick up little tid bits here and there. 2011 George H. Morris Horsemastership Training Session - USEF Network
I also find his critique columns in the Practicle Horseman Magazine very benefitial. I read them religiously. He also have DVD's out.
Also, I want to add that Sally Swift is very benefitial - if you want to jump, dressage/flatwork is a very important
aspect to the sport *afterall, jumping is dressage/flatwork with speed bumps* and learning balance is a very important ingredient in the whole recepie of jumping.
And the book "101 Dressage Exercises" that goes along with the Jumping Exercises book - to be extremely benefitial.
Many GP Jumpers spend the majority of their time doing dressage/flatwork and there's a good reason behind it. Because it is what is what you do between the fences that is more important, than the going over fences.