Yes, you can set a parameter for the minimum effective ppi, and the preflight will flag anything below that.
Open the Preflight panel (Window > Output > Preflight). Go to the panel menu and choose Define Profile. Duplicate the Basic profile by pressing the + button. In the IMAGES and OBJECTS section, check Image Resolution and set your preferred resolution. Save the preflight profile.
In your book, make sure the preflight profile is in your style source document. From the book panel menu, choose Preflight Book.
Thanks, that's exactly what I needed (and there were some other things I ended up checking in that list, as well).