The important RFCs and Internet Drafts relating to FTP are:
rfc959 The core standard.
rfc1123 Various clarifications and requirements.
rfc2389 Feature negotiation.
rfc2428 FTP extensions for IPv6 and NATs.
rfc2577 FTP security considerations.
rfc2640 Internationalization of the protocol.
draft-ietf-ftpext-mlst-11.txt Proposed extensions.
Other references of interest:
http://cr.yp.to/ftp/security.html