Delicious biscotti. The cooking time for the first bake might need tweaking. If the biscotti crumbles when you try to slice it, it's been overbaked on the first bake.
Ignore the 25 minute directive, and bake until the logs don't depress when lightly touched. They should be firm, but not hard. A very light touch is all that's needed. For me, at 350°, they took 30 mins, and I checked doneness after 20 mins, 25 mins, 27 mins, and 30 mins. It sounds like less cooking time would have been better for other reviewers. You need to watch them so they don't overbake. Wait the full 10 mins before slicing.
Tip: Right before slicing, spritz the top of the log with a little lukewarm water; this adds a little moisture back to the logs, which will help keep them from crumbling.