The “Sieve Manifesto” aka Sieve White Paper, aka “The Sieve Language and a General Model for Delivery and Interoperable Filtering in Internet Mail” is available in PDF format here or in a somewhat uglier version in HTML.
This is version 2.00, updated in February, 2001 to reflect the evolution of Sieve to date.
There was a Network World article on the front page in August, 2000.
Slides were prepared by Tim Showalter for the formal Sieve BOF at the 41st IETF at Los Angeles, California, describing Sieve Syntax. They're here in PostScript format. Please note that portions of this Sieve syntax are now obsolete, and that the complete presentation was not made at this meeting due to time constraints (see also the minutes for that meeting, linked in the history and milestones section.
There is a brief description of SIEVE in the O'Reilly Managing IMAP book by Dianna & Kevin Mullet.