I have "Professional SQL Server 2005 Integration Services". I have not used the book extensively as I am currently extracting from an Oracle source into SQL 2005 and then using stored procs to carry out the ETL.
Most of my advanced work is in SSAS and I would recommend "Applied Microsoft Analysis Services 2005" by Teo Lachev.
I agree both books listed previously are excellent. You should also look at "The Microsoft Data Warehouse Toolkit" from Wiley. Much of the book is devoted to SSIS.
Frank
I agree that all three books are must haves.
The MS Data Warehouse toolkit is particularly good if you haven't built a warehouse before, because it concentrates much more on the WHATS rather than the HOWS.
You may also want to consider "The Rational Guide to Scripting SQL Server 2005 Integration Services'.
Its not a must have, but can come in handy and is a quick read.
The book is great and just attended the class taught by the authors of the MS Datawarehous Toolkit.
Highly recommended.
Mark