The SFP, or Small Form-factor Plugable and it's updated brother, SFP+, are part of the SFF alliance.
The first SFP specification was guarded by the SFP-MSA, SFP-Transceiver MultiSource Agreement. They produced a 'basic' spec, following up from the GBIC specification.
SFP
The SFP-MSA spec later renamed to INF-8074i, though the document is the same, part of 4 introductory pages. It contains the mechanical, electrical and ID specifications.
SFP+
The SFP+ spec is named SFF-8431 and sits at revision 4.1 since 2009. It only contains the mechanical, electrical and electrical testing part of the specification. It also refers to other SFF specs, one being the Management Interface for SFP+ as SFF-8472 and contains EEPROM layout (at 0x50) and a diagnostics and monitoring interface (at 0x51), that is read like a dynamic EEPROM.
https://www.snia.org/sff/specifications2 for all specs, requires login!