I think there's a false dichotomy here. Memorising a dictionary is easy, because memorisation is a choice (with spaced repetition). Reading a book in an unfamiliar language takes lots of time and mental effort. Both will contribute to your understanding of the language; perhaps memorising a dictionary contributes less, but then there's a lower activation energy involved in it too: whenever you have five minutes spare, you can just sit down and learn a couple of cards.

If you use vocab flashcards that contain an example sentence or two showing usage in context, then it is more efficient than reading a book (with regard to the language rather than the book's actual content). Personally, I read books and when I encounter an unfamiliar word, I add it to the flashcard pile, ensuring that I don't forget the new knowledge.

If you're trying to learn the content of the book, the process of converting the information into flashcards entails engaging with the material, so that isn't skipped.

No one memorizes dictionaries for language learning.

There are frequency lists of vocabulary words for languages: basically some words are used more frequently than others so memorizing the most common 1000-3000 words is enough to get very high comprehension rates for everyday conversation and reading.

https://blog.fluent-forever.com/vocabulary/