What type of book did your wife write?
She could enter awards, submit to book-review sites, write articles directing people back to her book's sales page, hang out with other writers (who are always avid readers), join a book community where they sell each other's stuff.... there are heaps of options
I have written 7 ebooks and they sell very well. The sales and promotion for them are all very different - because they're all in completely different niches.
3 of these are based on debt management, budgeting/saving money and investing. 3 different topics. 3 different approaches.
I asked various finance website owners to review these copies. Visitors trust reviews. I made sure copies were available on Amazon and made sure those site owners could earn a commission if they sold any of my ebooks to their visitors.
The other 4 are completely different again. Sales are doing fine
