I'm not familiar with the mathematics taught in middle school in Poland, however, for a complete introduction to algebra, and a continuation to more complex mathematics, I would recommend HELM (Helping Engineers Learn Mathematics); it was originally designed to be used during the first year of university when studying engineering disciplines, as a means of teaching those whose knowledge of highschool mathematics was insufficient for university. The format is that of short workbooks; a full explanation is presented, but the majority of the teaching is via worked examples, and as far as I recall full solutions are presented for any questions asked. The first workbook covers notation fully, and start with very simple algebraic concepts, essentially easing into the abstraction from specific numbers to general variables, showing that you can cancel variables etc. - really the most basic of things. Subsequent workbooks cover algebra, linear algebra (matrices), probability distributions, numerical methods, optimisation problems, calculus etc. PDFs of the workbooks are publically available from the University of Southampton: http://www.personal.soton.ac.uk/jav/soton/HELM/helm_workbooks.html
