[landhar]

IMHO, the answer to this question is to frequently ask yourself this question as you're working on things. There's no perfect balance, you'll have to make a decision depending on the context and priorities of the moment. It might be ok to gloss over a concept you're unfamiliar with if debugging a production issue and having evidence that the root cause of the bug is in another area of the code altogether. But then make some time during feature development (or when writing a COE about the production issue) to go back to that piece of code and dig deeper into why it's written that way.

To expand on the analogy of the OP with regards to unfamiliar words in a novel, I personally don't look up every single word that I come across when I can get an idea of what it means from the context, but only after I encounter it a few times.

[tiogemini]

Good points. Thinking about the unfamiliar words analogy I would look up a word if I see it often enough or if it's key to understanding. Knowing if an unfamiliar word is important is tricky though--I can't define the process I use there.