Invisible is an Ivy Malone Mystery, the first in the series. It's a great book and it's still free.
Ivy Malone is a late bloomer in the detecting game. She's officially a senior citizen, but now that she's retired from the librarian business and a widow she's taking some chances. The fact that she feels like an invisible little old lady (LOL) gives her a boost. She starts investigating vandals in the cemetery and quickly moves up to murder. Throw in a little romance and you have a great story. Let me warn you there's something of a cliff hanger, but the first book has a satisfying conclusion in spite of that. Very highly recommended.