Sleisinger and Fordtran GI and Liver Disease is the primary text book. If you are looking for stuff that is beyond textbooks, all the societies produce guidelines. Look at the IBD guidelines in AJG, the endoscopy guidelines on the ASGE website and the hepatology guidelines on the AASLD website. If this is preparing for fellowship, that would be a good start. If this is preparing for a rotation, relax, its GI.