Jim Minatel of Wiley has a good post on plagarism. It is at http://wroxblog.typepad.com/minatel/2006/11/an_authors_view.html. His post is built on Nicholas Zacharias' post at http://www.nczonline.net/archive/2006/10/385.
The bottom line is that it is important to not read someone else's work
when you are working on the same subject. I remember a couple of years
ago, I was talking to Bob Beachemin. We were talking about Sql Server
CLR Objects. he offered to send me a copy of his book on Sql Server
Yukon beta2. While I have always appreciated it, I told him at the
time it was probably not a good idea because I was working on our
ADO.NET book and specifically the part on Sql Server CLR Objects.
So, the bottom line is that I agree with Jim 100% on his statements.