Well,I know that JLS is pretty dry reading,and as Corey said,its great to be used as a reference,but I read it straight through before giving the exam.
The thing is that if you read JLS,then you will get great marks in your mock exams,and that will raise your confidence for the real exam.But be careful,it is not very interesting to read,so try it,and if you can bear it,then read it!