Today I was wondering about the real difference between the words "book" and "tome", and I found two things. The first answer is that a tome is usually considerably larger and heavier than a book. But the second answer, (and likely the better one) is that a tome is usually a compilation of several books. For instance, The Three Books of Occult Philosophy by Eliphas Levi would be a tome, as it is three books combined into one volume. My grimoire would also be a tome (yay!) because it is several books combined into one tome. (The book of foundation, the book of light, etc)
I love having mysteries cleared up!