Unfortunately, there is no easy remedy for this problem once it has occurred. Prevention is the best solution and keeping your books upright with the proper support on either side will work to stop this problem.
Sometimes if the leaning is not too bad, flexing the binding in the opposite direction will lessen the problem. With worse cases of leaning, this is usually not successful. The best way to stop this is to have the book rebound, which can be rather expensive. Only by removing the binding and the layers which stiffen the spine of the textblock, then straightening the book out, the relining the spine and reattaching the cover can you be sure the book will stand straight.