The need for a licence will vary from municipality to municipality.
Check with your local city hall.
In some communities she will need an "entertainer's licence" in others she will not need to get anything at all.
I have in the past found it is smarter and easier to check first than to run afoul of local bylaws.
If this is a one time thing, as in a promotional thing done by the bookstore, it is up to them to be sure the paperwork is in order. It maybe that she is being paid by the bookstore as a freelance employee, and that definitely puts the responsibility squarely on their shoulders.
If however, the bookstore is allowing your wife to use their property to regularly supplement her income, then it is her responsibility to look into the need and acquire any and all permits and licences.