Copied this down years ago:
The procedure I am about to describe will result in a map book about the size of a Sports Illustrated magazine.
First thing: Make sure you have an equal number of maps to fold. If you do not, get some heavy stock paper the same size of the map sheets to round out the pile of maps. Trim the maps of any excess paper you do not need, but be sure to trim them all to the same size and leave a little extra area in the margins for slop and grid markings
Second: Lay the first map out with North at the top and the printed side up. Bring the East and West edges together so that you have a crease running North/South. Unfold the map and turn it over so the printed side is facing down and North is pointing up. Bring the North and South edges together so that you have an East/West crease and you are looking at the South edge of the map sheet with the printed side upside down. Pull the South edge to the center crease, creating another East/West crease. Do the same with the North edge. When you are done, turn the map over with North at the top and the printed side up. You should see 3 East/West creases and one North/South crease. Repeat this procedure for each map sheet.
Third: Place glue (gluesticks work REALLY well for this) on the back side of each map sheet so that the back sides of the middle panels (North and South of the middle East/West) crease are stuck together. Repeat for each map sheet.
Fourth: Arrange your folded and glued mapsheets so that all of the maps that are North and South of each other are lined up that way. Place glue on the bottom panel of the Northern-most map and the top panel of the next map to the South. Continue this process until you have completed each string of North/South Mapsheets.
Fifth: Take each string and find the middle point where you have a relatively equal number of panels North and South of the East/West crease. Fold each string along the North/South crease. Starting from the Western-most string, place glue so that the right side gets glued to the left side of the next string to the East. When you are finished, pace the whole assembly on a flat surface and put a heavy flat object on top. Let it dry for a few hours.
Sixth: Once the whole thing is dry and any touch-up glueing and trimming is complete, you will probably want to put on a cover. The first thing I have always done is to take some duct tape and create a spine North and South. You don't want to compress the maps too much when you do this or the whole thing will try to stay open after you put the tape on. If every thing dried OK under the heavy flat object, the maps should be at a good thickness to put the tape on.
Next take some relatively flexible and sturdy material, such as posterboard, and cut out two pieces slightly larger than the maps. The idea is to have a slight edge that protects the map. Glue them on. If you saved any thing from the trimmings, such as legends or scales, you can glue these on the outside. Cover the outside with some plastic and you should be good to go!
There is a way to make an even tighter and smaller fold, but you end up with something that is about the same size and shape of a FAR/AIM book. I have never done it. Pictures are worth a thousand words, but I don't have any. I recommend you practice with a few pieces of paper before you start trying this technique on your maps.