Is the book useful for learning only Object Pascal, or it can be somewhat used for learning Lazarus as well?
As the name implies, it's useful for learning Object Pascal (in fact the "Delphi" language).
As an aside: people often confuse (Object) Pascal, the language; FPC, the Free Pascal
Compiler; Lazarus. the IDE, and the LCL, the Lazarus Components Library; but they are quite different beasts, so while a book about one of them might cite any of the others it shouldn't mix them.
Thus, a "Lazarus" book should basically be like, say, a book about Word or WordPerfect, though the examples must, of course, use code written in Pascal; while a book about Pascal might cite and/or use FPC/Lazarus as the tools of choice but hte emphasis should be put in the language itself. IMHO, of course