Instead of a book, have you checked out the groups that sell patterns to download?
They have all levels of bags, with good instructions and some have video sewalongs/tutorials.
There are a few free patterns to download, and lots of inspiration from other bag makers.
RLR Creations, Bagstock, Sacotin, ChrisW Designs, Blue Calla to mention a few.
Even just browsing what others make, gives me lots of ideas.
(Just seen the post that Lizzielogs started on free bag patterns, that has links to some of the sites)