Character secrets were introduced as a fully-fledged character concept for 5th Edition in Icewind Dale: Rime of the Frostmaiden, but Eberron Confidential goes even further than that. This is a supplement providing a massive list of over fifty character secrets designed to be taken by players at character creation to enhance their connection to the world, while at the same time serving as character hooks for the DM to work with and providing player characters with small mechanical benefits. While most secrets are conceived of as things to be taken at character creation, as a DM I'm eager to try and implement them into a long-running campaign set in Eberron. Some are easier than others to introduce during an ongong campaign (hello, Creepy Doll!), but I think part of the fun as a DM is trying one's best to do so with any character secret a player wants, regardless of which secret and what stage of gameplay.
It would have been very easy for Eberron Confidential to simply provide a list of player-facing character secrets and leave it at that, but the supplement goes even further. Each secret gets specific tips and suggestions for the DM to integrate them into their campaign, and these are as fun as the secret itself! In addition, the rumors and random secret-generation tables for NPCs are so good that I want to add a secret to every major NPC I have now! They're just that compelling.
The secrets in Eberron Confidential range from the funny to the creepy to the absolutely world-shaking and epic, and they will definitely become a standard part of character creation for my games set in Eberron from now on. I rarely run other settings, but many of the secrets are setting agnostic, and others can be used in other settings with minimal changes. Quite a few are inextricably linked to Eberron as a setting, however, such as the Secrets of Sharn section or those secrets with an Eberron-specific race as a prerequisite. These would be difficult to work in other settings, but to me as an Eberron lover, that is a function and not flaw of the supplement. An absolute must-have for fans of Eberron!