I call my new sketchbook The Montanapolitan Croptic.
Montana, because it is held together with dental floss, portmanteau-ed with Neapolitan, in honor of its three flavors of paper (plain, toned, and watercolor) and Croptic because its Coptic stitching is cryptically concealed on the double page spreads.
The toned paper signature, showing the threadless page gutter.
A handy pocket for your Hor:ratio or other accessory.
I used Canson Biggie Drawing pad paper for the plain pages. It takes the Noodler's Konrad/ Noodler's black combo quite nicely. The toned drawing is from the Strathmore Toned Tan pad, and the watercolor paper is Arches 140 cold pressed. The dimensions are 6" x 4", which I like as a format and which I could get from the 12" x 16" sheet of relatively expensive Arches with no waste. I had to lose bits of the Canson and Strathmore papers to match the format, but the cost was less.
I bound the three signatures with the modified "cryptic" Coptic stitch and covered it with fabric from an old pair of black Wrangler jeans. It turns out that the denim isn't the best for this application - it unravels easily and is hard to cut straight, but once it's all glued in it seems to work OK.
There are many references for the basics of coptic stitching and sketchbook construction - here are some excellent ones:
Do the regular Coptic stitch binding, but instead of a full page in the middle of each signature put a narrow strip of paper and sew that in, as if it was another page, then glue the middle page to the strip. Now there are no threads or holes in the middle page, so you can draw right across the double spread. There is still a bit of crease or gutter, but that's livable with.
Here are the components for a simple version with only two signatures. The signatures are already punched, along with their center strips (which also make handy templates for the punch). Below them are the the pristine, unpunched middle pages, and the covers.
The signatures, covers and tapes have now been sewn with the Coptic stitch and waxed dental floss. You could also use special bookbinding thread, or plain thread with wax on it. Since this photo was taken, I learned that a curved needle makes stitching easier.
The strip is glued. I used archival PVA glue, but regular Elmer's or library paste would probably work fine. The extra paper tucked behind is to stop any glue from getting on the pages.
Here it is - the silky smooth inner page, with the messy threads out of sight beneath it.
From behind, the strip is visible and takes up part of other side of each page. This to me is a plus - it reminds you that if you draw there it could wind up being on the verso side of your centerfold drawing. This actually happened to me - someone wanted to buy and frame a centerfold painting I did, but there was another painting on the verso that I didn't want to hide behind the mat, so I built a special frame with glass on both sides.