Bookbinding 101: Rulers
Our workhorse rulers are two acrylic quilting rulers, a 6" x 24" and an 8.5" x 24" Omnigrid—or for the rest of the world, 15.24 x 60.96 cm, and 21.59 x 60.96 cm. There are other brands that would work equally as well.
We primarily like them because it's faster to measure length and width at the same time, and they also help to keep our cuts square. Additionally, their large surface area gives more friction to aid in their not sliding (but they can still do so), and the measurement lines on them are ever so slightly raised/grippy, which also helps them to not slide on fabric and leather.
The width of these rulers work well for a lot of different sized books, but there are times when we find a 12.5" square or 15" square ruler would be nice to have. Or even larger. But we definitely get by with the 6" and 8.5" widths, as their 24" length serves us just fine in most cases.
Quilting rulers work wonderfully with rotary cutters, but when using a fixed blade knife to cut along their edge, it's possible to accidentally carve out a bit of the acrylic edge, so it helps to take a little extra care to not turn the blade into the ruler when cutting. Our rulers have a few nicks of their own from our less careful use of fixed blades when using them, but we can't recall cutting into them when using a rotary blade. It is possible that either kind of blade can jump the edge onto the surface where your hand is holding the ruler down, so take care to keep your fingers away from the edge, or wear leather gloves, or both.
Steel rulers are great at avoiding nicks by your fixed blades. If they're cork backed they are especially good for not slipping, but they still can slip. Because we have the wider quilting rulers our steel rulers don't see much use due to their being limited to measuring and cutting the width, and then separately measuring to cut the length. Gets the job done, but greater chance of being less than square, and not quite as quick. Another thing to be aware of, as pointed out by Elissa of Blue Roof Designs, is that the cork on cork backed rulers lifts the steel ruler up off of the surface of your material, and your blade can angle underneath the ruler, cutting off more than you intended. It may not be reason to ditch the cork backed ruler, but it is good reason to take extra care to keep your blade vertical when cutting.