I recommend Ordinary Differential Equations by Pollard and Tenenbaum. It's a Dover book--only costs about 20 bucks too. Besides the vast amount of info on ODE's, the sections on Laplace Transforms are the best. I would advise against using Differential Equations by Edwards and Penney for studying Laplace Transforms, because in my opinion, the section on them was incoherent and wasn't explained very clearly.