If you are looking for information on the Modbus protocol, the best source is the modbus.org web-site. You could also try the Modbus users' community site, particularly if you have a specific question that you want to ask.
Gould-Modicon/Schneider/modbus.org have published five documents describing the Modbus protocol:
Modbus Protocol Reference Guide. PI-MBUS-300. Rev J. June 1996. This is the original Modbus protocol specification, which has been superceded by the "Modbus Application Protocol" and "Modbus over Serial Line" documents listed below.
Open Modbus/TCP Specification. Andy Swales. Release 1.0. March 1999. This document defines the Modbus/TCP protocol, which is used to send Modbus messsages over a network using the TCP/IP protocols. It also contains some intelligent commentary on the Modbus protocol, and an appendix on client and server implementation, which may be useful if this area is new to you. Surprisingly, this document does not appear to be available from the modbus.org web-site.
Modbus Application Protocol. V1.1b3. April 2012. This is the current definition of Modbus function codes and the format of the corresponding request and response messages. It does not cover how these messages are packaged in the RTU, ASCII or TCP variants of the protocol.
Modbus Messaging on TCP/IP Implementation Guide. Rev 1.0b. December 2006. This consists mainly of what appear to be design notes for Schneider Automation's implementation of Modbus/TCP. You will probably find it hard to read if you don't already have a good understanding of the area, and not very useful if you do. It is, however, the only document on the modbus.org web-site that contains (in section 3.1) a description of the Modbus/TCP protocol.
Modbus over Serial Line - Specification & Implementation Guide. V1.02. December 2006. This is the current definition of the RTU and ASCII variants of the Modbus protocol. It also contains physical layer (electrical) specifications.There are many non-standard extensions of the Modbus protocol. One of the most common is that introduced by Daniel Flow Products Inc (now a subsidiary of Emerson), and often called "Enron Modbus". There is no single document describing the Daniel Modbus protocol extension, so you have to refer to documentation on specific products such as Daniel: Modbus Communications Model 2500 or ABB: Enron Modbus Protocol for CB181 FCUs.