does the book contain also documentation about Object Pascal or just about working with Lazarus ?
From what I can understand in the German table of contents, there are about 20 pages in the book dedicated to the Object Pascal language. This means a very concise summary at best.
You are correct, in Chapter 1 there are about 20 pages which form a very concise summary about Object Pascal. But it does cover a lot of parts, even if quickly in each of them. It covers most basic types (integers, floating-points, c-compatible integers, records, sets, arrays, pointers, etc), function declaration, constants, the layout of a unit, basic RTL units, object orientation, memory allocation and the Format function.
In Chapter 6 there is a short 10-page summary about which classes and units are available in the Free Pascal RTL and FCL.
Various other parts of Free Pascal RTL and FCL are described in places like:
* Chapter 5 describes a little bit of platform-specific programming for each platform (Windows, Linux, Mac OS X)
* Chapter 8 covers, among other things, how read and write files, using the basic routines (Assign, ReWrite, Reset, WriteLn, ReadLn, Write, Read, etc), as well as with Streams and how to connect both methods.
* Chapter 10 explains threading
* Chapter 4 explains the different project types that can be created with Lazarus and so it explains a little bit of each. For example, how to create a DLL library.