Two main documents are officially maintained by the Coq development team:

• the Reference Manual, which is the complete, authoritative source of documentation for Coq;
• the documentation of the Standard Library distributed with the system.

PDF files and html tarballs are here.

Moreover, some users of Coq have published various documents that may be useful both to beginners and advanced users of Coq:

• the Coq'Art, the first (and only) book dedicated to the Coq proof assistant (Yves Bertot, Pierre Castéran, 2004, Chinese version in 2009);
• Software Foundations, a Coq-based textbook on logic, functional programming and foundations of programming languages (Benjamin Pierce et al, 2007, with regular updates);
• Certified Programming with Dependent Types, a textbook about practical engineering with Coq (Adam Chlipala, 2008, with regular updates);
• Video tutorials (Andrej Bauer, 2011);
• Coq in a Hurry (Yves Bertot, 2006);
• an old Tutorial, which is a commented interactive session which introduces basics on terms, proofs, and tactics (Gérard Huet, Gilles Kahn and Christine Paulin-Mohring);
• a Tutorial on Recursive Types in Coq (Eduardo Giménez, Pierre Castéran, 2006) and the associated examples;
• a bunch of Frequently Asked Questions;
• Cocorico!, the Coq wiki.