I did this by using cheap hardboard from some clipboards, and some posts with screws.
Step 1: Ingredients
Two legal sized plain old hardboard clipboards
Fabric for the front sides
Fabric for the hinge on the inside
Two sheets of cool paper for the insides
Bits of leather scrap if you are messy on the corners like me
Glue. I used wood glue because it was sitting around.
Three internally threaded posts with screws I found at Lowes.
I had priced 1/8th inch hardboard at Lowes, and it was under $10 for an 8' by 4' sheet. The problem is I can't get that home easily. The clipboards had the added advantage of being near the right size. I only need three cuts each. I'm sure this would work with just about anything board-like, though.
Step 2: The Boards
Since I am using standard 8.5 x 11 inch printer paper sheets inside, I made the spine part 1 1/8th inch by 11 1/2 inches, and the cover 7 3/4 inch by 11 1/2 inches.
I planned on a 1/4 inch space between the two pieces that would be covered with fabric to act as a hinge.
Since my neighbors are getting tired of hearing my power tools, and it's just 1/8th inch thick hardboard, I just used a hand saw.
I then sanded the edges smooth.
Step 3: Fabric Covers
I used some old exterior upholstery canvas.
Step 4: Glue, Glue, and More Glue
Then, when the boards are glued down, wrap the edge pieces around and glue them down.
Let them dry.
I'd like to give some advice on the corners, but mine didn't come out very nice. My fabric is very very thick, and just wouldn't lay right. I'm sure there are some fine bookbinding instructables here that will show you far better than I could, anyways. I also could not manage a camera with the glue, the stiff fabric, and the cat at that point, so my apologies.
Step 5: Holes
I then lined up the paper on the boards, made sure they were all where I wanted them. I then drilled through the holes. I made sure the drill bit I used was only slightly bigger than the post.
Step 6: Inner Hinge
Don't worry too much about the fabric, because your lining paper will cover this anyways. The sheet fabric sort of matched what I was working with, so I used this.
Step 7: Interior Paper
I used a hole punch to punch out the holes where i needed them to be.
Step 8: Corners
I drew a 1" x 1" square on the leather, and sewed it all down. Then I crew a line catty corner across, and cut it.
I then flipped them all inside out, and glued them on, using clamps to secure them.
Step 9: Finishing
The important thing is that I can unscrew the posts, and take out sheets, or add more. If my book gets thicker than the posts, I can buy longer posts. It works so nice, I am thinking I will do this for a binder next quarter for classes.
I added an embroidered Lovecraftian elder sign on the front. It fits the nature of the character whose slide into madness is being captured by this grimoire.