Hi Vagelis,
Although I don't know any books in Greek, you should try and find a book by DOEBELIN called "instrument systems"- lots of good stuff in there.
There are two kinds of encoder, incremental and absolute, incremental encoders send pulses as they rotate that only contain the fact that they have moved a certain increment, and which direction they have turned. Many incremental encoders also include an index pulse so that you can find a fixed start position. They send two channels of pulses 90 degrees of phase apart. You can deduce the direction of rotation from the phase.

Take a PCmouse apart and see two incremental encoders inside. A 2000 PPR encoder sends 2000 Pulses Per Revolution from each channel. Since there a two edges from each pulse on each channel by clever logic (actually just an XOR gate, you can get 4X the nominal resolution.

Absolute encoders give an actual position in absolute units of binary angle,
They rely on a coded disk with a binary "grey" code etched into it which is often read out by optical means .

These can be very expensive indeed

Steve