One of the rule of thumb that I always use when picking a book is to flip to a random page and read through it. If there are more than 5 new vocabularies, that means that that book is slightly too difficult for my reading level.
Also, I do not like using the dictionary. I am more comfortable with trying to interpret the meaning of a word by putting it in the text it comes from. I feel that looking a word up in the dictionary distracts me from the flow of reading.