Mercer is the lead writer for the "Explorer's Guide to Wildemount", a campaign setting book for "Dungeons & Dragons" 5th edition based on the world of Exandria, home to the Critical Role campaigns. This book details the geography, cultures, and lore of Wildemount, and it includes new player options, monsters, and adventure hooks.

"Tal'Dorei Reborn" is a comprehensive campaign setting guide for "Dungeons & Dragons" 5th edition, focusing on the Tal'Dorei continent from the first Critical Role campaign. Mercer supervised the project in conjunction with Darrington Press.

Beyond published materials, Mercer has heavily adapted and customized D&D rules and settings for his own games. He is known to make extensive homebrew rules to improve gameplay.