
I'm always looking for new ways to experiment with bookbinding. One of the ideas that occurred to me was to make a double-fold book that could be made with readily available supplies; like composition notebooks.
When I was younger, composition notebooks were worth their weight in silver. They were handy for just about everything. About two years ago, I found that a dollar-store near me was selling these pocket-sized composition notebooks and I picked up a few dozen to play with.
This project involves turning two of these small,

comp. books into a double-fold book. I've made several full-sized, double-folds for friends over the years and this is just an easier, quick version. Double-folds are handy because it allows you to have two separate books within the same cover. I have used the double-fold design for things like bibles, diaries, and scrapbooks. It's a handy design because you can take a finished book (in the case of a bible) and add a notebook to it for your own research or reference. I didn't like cutting the cover of the bible (I'm a devout follower of "The Book" - aka bibliophile), but the friend I was making the piece for didn't mind as it was one of the free copies of the Book of Mormon.
STEP 1:Stand book 2 (your secondary book) up on its edge and trace it against the inside cover of book 1 (the main book you'll be writing in most of the time). Trace the width of Book 2 and give it a little bit of wiggle room. You DON'T want it too tight.
I use a clip to hold the second book upright for the picture, but you won't need anything like that. Just use a pencil or a pen to trace the width.
(* Book 1 is the book that you will constantly be referencing or writing in. In the case of the Double-Fold Bible, I put the bible there since I knew that my friend would be reading it a lot. Book 2 would be where he would write his questions and comments.
STEP 2: 
With a sturdy ruler and an razor-knife, cut Book 1 at the line you just traced. I used a small cutting board under the book since I didn't want my desk marked up.
It's very important to leave plenty of room on this cut.
If you plan on adding pictures to Book 1 (for a travel-journal or perhaps a scrapbook), you will want to add half the width that you measured so that when the pages bulk up it won't strain the spine that you're creating.

When you place the two books together, there should be a little bit of an over-hang of Book 1's cut cover.
STEP 3: To connect Book 1 and Book 2 you need a flexible binder. I used the Handy Man's Secret Weapon: Duct Tape. You could use cloth tape (aka: athletic tape) if you wanted a different look on the spine.
Scotch Tape, Masking Tape or anything that's cellophane-based won't work as its flexibility will eventually wear out.
Step 4: 
Cut the strip of Duct-Tape in half (by width) and apply it to connect the back cover of book 2 and the cut cover of book 1. However, you want to leave a little "wiggle" room here as well so the books open smoothly.
COMPLETE: Once you have taped the two books together it should look something like this.
The spine of Book 2 should line up with the edges of the pages of Book 1. If you wanted, you could attach some kind of closure mechanism (elastic or a tie) to hold the two books closed if you want.
For the purposes of this project I didn't add any closure as the books are so small they'll not need any. For a full-scale double-fold, I would recommend an elastic band around the middle or on the edge (like a Moleskine)
If you don't like what the inside of the newly bound double-fold looks like, I would recommend that you add a decorative end-page that would stretch from the last page of Book 2 to the first page of Book 1.

For the purposes of this project I didn't bother with covering the composition books. If I were finishing the project completely, I would grab construction or scrapbooking paper to cover it.
For more information on how to recover a notebook, see my
earlier post.
Edit (6/20/09): The concept of the double-fold notebook can be used for a number of different functions. Double-fold books are a few-hundred years old by design. They were originally created to enclose both a journal and reference book under one cover. The most commonly found example is that of a medical journal.
So let's say that you have two books in one: the book on the left (often folded up against the one on the right) is not accessed as often. In medical journals of the time, it was where the physician (or other scientist) would record their proven formulas or concoctions that were effectual against given symptoms. The book on the right, more commonly accessed because of the design, would be a series of running entries about what medical cases they came across; essentially a "chart" for all of their clients as they were seen.
If you were using it for research, the left book could be used to record all of your formula or other established tools for your work and the other could be used to record your progress.
Good luck.