Of course, if you are really serious about learning the words in an active way, use an SRS (Spaced Repetition), Word lists or whatever method you prefer. This is, however, not something you automatically have to do just because you study the language in which you are reading a book. A lot of words will stick anyway. If you do choose to make more of an effort, be careful with classics. Before you spend time on memorizing 50 new words, make sure they are actually in use today and not something that was scrapped two centuries ago.