The L1 DLP study programme has been created around the set books, which present the original method as laid down by Dr Bach. So the answer is, yes, you do need to have the two set books even if you already own or have read other books.
Some popular books on the remedies contain errors and others are based on personal reinterpretations of Dr Bach’s work. You may find your progress unsatisfactory if you try to rely on other texts.