Thisis also described in the Make weekend project 12
Wood glue and a book - hard-cover is best. I got this one for five francs at the flea-market last week-end. Next, dilute your wood-glue with water, about 50-50 should do. You want it quite runny, but still thick enough to glue the pages together.
Decide on the depth of your secret compartement, and put a blastic sheet between that and the rest of the book. Apply the wood-glue with a brush on the sides of your compartment. Then put some weight on your book (so the pages don't warp) and let dry.
Next, open up the book, get an exacto-knife and cut out your compartment.
Get your trusty glue again, and glue together the walls of your compartment on the inside. Let dry under weight again.
Here is the end-result, the iBook! Now I can pretend to read bad french poetry from the 19th century, while secretly reading the latest fantasy-trash. This is of course only a first try, I still haven't given up on making an iBible!