This is the third book in the Bookshop on the Shore series, but it can be read as a standalone. However, you'll miss the wonderful character connections that make Colgan's books so superb.
I always like returning to a place in a book that I've visited before and getting caught up. I really enjoyed both Lissa and Cormac and their adventures both apart and together. Their very real dealings with PTSD, but also the heartwarming connections that both of them learn to make when they are forced outside of their comfort zones. If you're looking for a book with great characters, some romance, a bit of humor, and all over good story, then Colgan's books are definitely ones to pick up.