So this is an interesting one...
Old favorite: Books for reading
New favorite: Books for planters
These particular books can be purchased as a kit from this website but I'm sure it would be a super cool DIY.
-Old book (please use one that is not a first edition or rare book...pretty please, I hate when I see amazing books turned into art projects).
-Some sort of plastic planter that could be used to line the inside of the book.
-Glue-ish substance, maybe mod podge to hold all the book pages together.
If you make one please comment with the link, I'd love to see it.