"The 4+1 View Model of Architecture", by Philippe Krutchen, IEEE Software, November 1995 - One of the definitive works on this topic. This is available on-line via the IEEE's Computer Society website at http://www.computer.org in the Digital Library. It is also available thru the ACM's digital library at http://portal.acm.org. You may also be able to find PDF scans of this document by doing a Google - especially of .edu sites since it is so often included as a course handout.
"Documenting Software Architectures: Views and Beyond" by Clements et al, 2003. Covers the more general topic of documenting software architectures but includes discussion on using UML, the 4+1 Views, and related topics. See http://www.sei.cmu.edu/architecture/products_services/dsa_book.html
"Architectural manifesto: Designing software architectures, Part 5: Introducing the 4+1 view model" by Mikko Kontio, published on IBM developerWorks, 02 Feb 2005 - This is in the midst of a series on wireless development but still provides a good 1-2 page summary of the 4+1 views. See: http://www-128.ibm.com/developerworks/wireless/library/wi-arch11/
The "Handbook of Software Architecture" at http://www.booch.com/architecture/ by Grady Booch. Includes a 2004 presentation on software architecture that discusses software architecture in general and includes several slides on the 4+1 views. See: http://www.booch.com/architecture/blog/artifacts/Software%20Architecture.ppt