Thanks for all the responses. To answer some of the questions,
I did have a backing strip on the bending jig. I used nylon
webbing rather than leather as the book suggests, but I don't
think that'd make a difference. The ribs are 3/8" thick in the
centers with the ends thinned down to 5/16" to encourage them
to bend there (again, as the book suggests.) I steamed them
for 15 minutes.

My guess is that I misread the grain of the original board before
I milled it. I thought what I had was flat grain. As I understand
it that means I should rip the board, turn the ripped pieces 90
degrees and rip again to get vertical grain (as in the top of the
picture below.) If the board I start with is already vertical grain
it should be resawn to the rib thickness, then ripped to preserve
the vertical grain (as in the bottom picture below). I thought
it was flat grain so I ripped it first, but I think it was already
vertical grain and I should have resawn it first.

Ah well, it's a learning experience. I'll have to try again. Although
my wife gave me a funny look this morning when I told her I was going
to go out to try to find a nice piece of ash.

Chris