I See Us is the third book in the Oracle series, which is set in the same universe as the Dowser series.
While it is not necessary to read both series, the ideal reading order is as follows:
Cupcakes, Trinkets, and Other Deadly Magic (Dowser 1)
Trinkets, Treasures, and Other Bloody Magic (Dowser 2)
Treasures, Demons, and Other Black Magic (Dowser 3)
I See Me (Oracle 1)*
Shadows, Maps, and Other Ancient Magic (Dowser 4)
Maps, Artifacts, and Other Arcane Magic (Dowser 5)
I See You (Oracle 2)
Artifacts, Dragons, and Other Lethal Magic (Dowser 6)**
I See Us (Oracle 3)
Other books in the Dowser series to follow.
*Oracle 1 contains spoilers for Dowser 1, 2, and 3.
**Dowser 6 contains spoilers for Oracle 2.