Fungi of the Far Realms

From the publisher's website:

Fungi of the Far Realms is a guide to over 200 fantastical mushrooms. Each mushroom is lavishly illustrated in traditional watercolour and annotated by the esteemed fictional mycologist E. Q. Wintergarden, creating a diegetic field guide of such indulgent quality as to put most real world examples to shame.

The book is system agnostic, with a mild preference towards your typical fantasy RPG, but is robust enough for most uses you could think of. The majority of the book is the titular guide book "Fungi of the Far Realms" and the rest is mechanical assistance in using these mushrooms in your home games.