Modula-2

Modula-2 (as well as its predecessor Modula) is another language created by Niklaus Wirth and derived from Pascal (which came from Niklaus Wirth also).
It now uses - and this is new - in addition to the structures and statements already known from Pascal, a simple but well thought-out and valuable module concept,
something that one looks for in vain in many languages - even today!

The Books I read: Programmieren in Modula-2, written by
Niklaus Wirth, ISBN 3-540-13301-1 and 0-387-13301-1;
or in English: PROGRAMMING IN MODULA-2, also by
Niklaus Wirth, ISBN 3-540-12206-0 and 0-387-12206-0.

These are very good and detailed books that I actually can recommend to you!