well of course of you are trying to read a book I suppose you have at least a middle level. I think it depends of the kind of the book. E.g. if you read a book about cosmos or universe and I'd you don't know   much words of that

argument you'll find boring

If you like Harry Potter, you should try Laura Gallego.  Same type of literature, but by a native spanish writer.  I'm just finishing "Dos Velas Para El Diablo," a very fun read and easy to read without a dictionary, even by a "low intermediate" like me.

Next I'm going to try "El Enviado" by Jesus Vilches.  Fantasy literature is fun and easy, and you don't have to just go with translations of Harry Potter and Tolkien.

I started reading with fairy tales, or novels for children when I had just finished the beginner level. Then, when I was in intermediated level, I began to read the books that I had already read in Spanish, although I had to keep my dictionary right besides me, to look for the definitions or even sometimes the translation or checked on google images.

I think you should start reading from the beginning. But you should read basic books, because If you try to read a book with advanced vocabulary you'll spend much more time searching the words in the dictionary instead of enjoying the plot of the book.

If you are interested in Spanish fables and tales, try this ones: "La lechera", "Garbancito", "El pastor mentiroso", "El flautista de Hamelin"...

"El Alquimista" is very easy to understand and also "El principito" (The Little Prince), but I think that The Lord of The Rings which has been menitoned is too difficult to start. You will have to be looking for the meaning of words all the time... That's not reading, that's making a vocabulary list