The Compendium includes minor updated rules and setting information for the 7th Sea core rulebooks. It was offered free of charge to owners of the original printings of these core books.
From the back of the book:
This compilation of rules, updates and player information contains details on every aspect of 7th Sea. For new players, it provides an introductory guide to an exciting world of swordsmen and swashbucklers. For experienced players, it contains all the new material from the 7th Sea second printing, allowing you to stay caught up on everything. Whether you’re a bright-eyed newbie or a grizzled veteran, a salty pirate or a gallant musketeer, this book has what you’re looking for.
The 7th Sea Compendium contains:
-An updated history, expanded details on the Secret Societies, and new information on Objectionism and the Inquisition.
-Expanded details on the mysterious Syrneth, including rules for creating your own artifacts.
-A ship-building system, allowing your players to create new vessels for their Heroes.
-A quick-start Hero creation guide, compiled lists of character abilities, pictures of the powerful Lærdom runes, and more!