I have a question about reading a Spanish book side by side with the same book in English.

I purchased Harry Potter in Spanish and I was wondering the best way to read it. Should I read a sentence then read the same sentence in the English version? Or should I read a sentence in the Spanish book and look up the words I don't know in the dictionary instead?

I also have the audiobook, but I read somewhere that I should not listen to things that I don't understand 80% of. Will it harm my learning to listen to things in Spanish that I don't really understand?

Thank you so much.