RFCs:

[RFC 2458] Toward the PSTN/Internet Inter-Networking --Pre-PINT Implementations.

[RFC 2995] Pre-SPIRITS Implementations of PSTN-initiated Services.

[RFC 3136] The SPIRITS Architecture.

[RFC 3298] Service in the Public Switched Telephone Network/Intelligent Network (PSTN/IN) Requesting InTernet Service (SPIRITS) Protocol Requirements.

[RFC 3910] The SPIRITS (Services in PSTN requesting Internet Services) Protocol.

Publications:

[ISBN 0471356441] Converged Networks and Services: Internetworking IP and the PSTN.