Are there any free phosphorus-31 NMR spectral libraries?
There are quite a few it seems, a couple of examples are below:

University of Wisconsin Phosphorus NMR data, starts with a shifts overview.

Stanford's Magnetic Resonance Laboratory site P NMR at the SMRL, provides comparisons and a protocol for testing.

A general NMR site is NMR Wiki - has many links to many sources.