The Salt Cyclopedia of Comparative Mythology

Your average book on mythology will tell about, say, Hades, and in the course of it mention that he was a death divinity. If you want to learn about death divinities in general, then you've still got a lot of work ahead of you. 

This book approaches things from the opposite direction, which should be useful for worldbuilders. Rather than tell you about individual gods, this book explains how death divinities have worked across the world and then gives Hades and some other gods as examples. 

With The Salt Cyclopedia at your side, you have all the tools that you need to start building a pantheon that is fresh and full of new life at the same time that it feels real, as though it's an authentic mythological system that just hadn't been uncovered until now. It includes not only basic mythemes but also discussions on divine epithets, fusing multiple gods together, building non-standard portfolios, and more.