Questions on fs file handling for performance read-only access

The program I'm working on is going to end up with a set of read-only files containing collections of structure data. My original thought was to have the data split up into reasonably sized sets across a number of files, and load an entire file into memory at once, then switch files when a structure in a different file was needed (originally the need to access a different file would come maybe once every few minutes, nothing insane). Due to changes, I'm going to have to figure out a way to allo...