I would suggest reading without a dictionary. It's good for your brain and for general language learning skills development to try to infer the meaning of the unfamiliar words from context. You can highlight/underline these words and after reading a chapter or so, you can use the dictionary to write down their meaning. Checking every word on the dictionary while reading is really annoying. You'll get bored quickly and stop reading altogether. If it's too hard to understand the book from context I'd suggest either read a book you've already read or start with easy material that'll expand your vocabulary, like newspapers or magazines. You should keep in mind, though, that you'll always find some words you don't know when reading books in your second language. I've been reading in english for years and I'm also doing a degree completely in english and I still find unknown words in novels and/or textbooks. So try your best and keep your motivation high. Remember the first book is the hardest. Then it keeps getting easier and easier.