I usually just use contact cement. It's not uber-strong, but it gets the job done for most projects. It's best to stitch it as well though. On occasion I use CA glue when I need a quick fix to hold a small piece before riveting or stitching, however if you use just CA glue, the bond is actually stronger than the leather, and can tear the leather if much stress is placed on it. What's the project? Maybe that will give me a better idea of the best course of action.