Already, there are several books about Kylix, with more on the way.
Kylix: The Professional Developer's Guide and Reference, by Jon Shemitz. An excellent book (but I may be biased--I am the books' technical reviewer).
Kylix 2 Development, by Eric Whipple, Rick Ross, and Nick Hodges. I haven't see this yet.
Building Kylix Applications, by Cary Jensen and Loy Anderson. A good book.
Kylix Power Solutions, by Don Taylor, Jim Mischel, and Time Gentry. I haven't seen this yet.
Kylix Developer's Guide, by Charles Calvert, Marjorie Calvert, John Kaster, Bob Swart, David Intersimone. I haven't seen this yet.
Delphi/Kylix Internet Developer's Guide, by Lino Tadros, Lance Bullock, and Steve Teixeira. I haven't seen this yet.
Delphi books that might be helpful for the Kylix user:
Delphi in a Nutshell, by Ray Lischner. A reference manual for the Object Pascal language, which has changed little between Delphi and Kylix.
Mastering Delphi 6, by Marco Cantu. The latest edition of a classic.