This book does an excellent job of describing the core Delphi Object Pascal Language.

I have several other Delphi books that try to cover everything in Delphi. Usually they end up spreading the details of the base language throughout the book. With "Delphi in a Nutshell", I have all the details presented in a clear, easy to reference manner.

Now I don't have to open up sysutils.pas in the editor to find the prototype for StrPos (or was that StrScan? ;>