Jessie Weston's book is a great read, but, as I discovered thirty years ago when I drew on her ideas quite heavily in an undergraduate essay on something or other, was even then considered extremely unreliable; and would not, I suspect, be considered useful to serious study today except as an interesting side-light on thinking on the subject in the early years of the 20th century. It is, of course, essential to any study of The Waste Land, but that's another matter.