To Get The Full "Shanniverse" Experience:
Start Here:
From This Day Forward Duet
You Save Me
[Note: You Save Me can be read as a standalone]
Operation Duet
[Note: Operation Fit-ish and Operation Annulment can be read as standalones]
Silent Phoenix MC Series
[Note: The first five books of the SPMC series are written chronologically and must be read in order]
More in the SPMC Universe:
[Note: These can be read as standalones]