As others have said, jelly rolls do vary in the number of strips they contain. However, generally, they contain 40 strips, but that's not always they case. I've seen some 2.5" rolls which are less than 40 in count also referred to as "desert rolls" to avoid the confusion you had.
Charm packs (5"(usually) squares) fall into the same definition - they're usually packs of 40, but can be anywhere from 20 or so to more than 40.
Layer cakes (10" squares) seem to always contain 40 pieces, at last all the ones I have (about 30 so far) do.
Honey buns (1.5" strips) usually contain 40 strips as well.
BTW, that book you got is an excellent one, I have it as well, and have made several quilts from it. Those 2 authors have another similar book out called something like Jelly Rolls, Layer Cakes, and Charm Quilts, which has more great patterns using these packaged fabrics.