COMMUNITY LAW - PRINCIPLES - EQUALITY OF TREATMENT - DISCRIMINATION ON GROUNDS OF NATIONALITY - PROHIBITION - NATIONAL LEGISLATION ON BOOK PRICES - DIFFERENCE OF TREATMENT IN FAVOUR OF RE-IMPORTED BOOKS - PERMISSIBILITY ( EEC TREATY , ART . 7 ) FREE MOVEMENT OF GOODS - QUANTITATIVE RESTRICTIONS - MEASURES HAVING EQUIVALENT EFFECT - NATIONAL LEGISLATION ON BOOK PRICES - DIFFERENCE OF TREATMENT IN FAVOUR OF RE-IMPORTED BOOKS - PERMISSIBILITY ( EEC TREATY , ART . 30 )