Complex sentence: Much of this information was already available in mariner handbooks (also called "portolan" s), compiled by professional mariners and pilots, largely as a mnemonic device for their own personal use, and passed secretly from master to apprentice.
Split into simpler sentences: Much of this information was already available in mariner handbooks (also called "portolans"). These handbooks were compiled by professional mariners and pilots, largely as a mnemonic device for their own personal use, and passed secretly from master to apprentice.