I don't think you will be disappointed in that decision.  More knowledge can never be seen as a negative and the more prepared you are for this exam the better chance you stand of passing the first time so you don't have to take time, money, and effort to do it again. 

Keep going through the Code book of your choosing to be familiar with it as well.  Then, don't forget to download the Part 'B' book and get familiar with it and how it is different than D1.1.  Especially if D1.1 is your code book.  The more you know your way around both of them the better you will do on the exam because you can find things quicker and more accurately. 

Mainly, on the exam, don't forget to read ALL the answers before choosing one.  There will be several that have more than one correct answer, it is a matter of having the complete answer.  Using the complete sentence from the book and not just part of the sentence. 

He Is In Control, Have a Great Day,  Brent