To cut straight across a text block, a book-trimming knife must be ground only on one side ("singled beveled") and quite rigid, or it will meander over the course of the cut: like a chisel, except that it is used for slicing / draw cuts rather than push cuts. A very sharply angled skew chisel can work, but these are harder to find / more expensive than mass-produced flat chisels. A rounded chisel works even better, and it is easy to make one from a cheap flat chisel:
Power tools recommended! But you don't need a specialty tool: I used a plain ol' drill and a $6 (Shark, Miller, Forney) grinding wheel I found at the local hardware store awhile back:
See also Jesse Aston's, Glenn Malkin's, and Darryn A. Schneider's related videos.