Square up for trimming
If you were careful to square the book along the top edge, the other edges can be left rough. Artistic types like this effect because it proves that the book was hand made. I find these "deckeled" edges annoying.
There is a fun set of tools called a laying press, plough and tub that I'll probably make someday. For this small book, I used an ordinary wood chisel and a woodworking clamp. A clean woodworking vise would work even better.
UPDATE: I finally made a small laying press. It cost very little and took about one weekend to complete. I didn't write up the construction details, but if you're into woodworking, the steps should be obvious. Here is a brief description and photos.
I had to sharpen the bejesus out of the chisel blade using a roller jig and a sequence of ever-finer oil stones. Between stones, I lapped the back of the blade flat using a diamond plate. This removes the burrs. On the last and finest stone, the blade is tilted up by fractions of a degree to produce a micro-bevel. I finished by dragging the blade backward over a flat leather strop charged with buffing compound, still using the roller to hold the blade angle. After that, I was able to shave hair off my arm effortlessly.
I trimmed the top edge first because it was already pretty flat. A backing board is used to protect the clamp.
Press the chisel down firmly on the flat side and slide the blade along the paper at a slight angle to keep the leading corner from digging in. Only a few light cuts are necessary.
You'll know you've got the blade sharpened right when you see the trimmed book edge shines with reflected light.
Next I trimmed the outer edge. I used the small square to keep the edge at right angles to the previously trimmed edge.
Repeat the process on the bottom edge.
