This novel is important for three main reasons. First, you understand better Mazoku's plan and what they want to create. Second, the boss is really amazing and, personnally,I hadn't found who it was until the end. The battle is also nice because each character plays a part in the victory. Third, Gourry "finds" his new magic sword and it's a really good one :).
So, even if it's not the best, it's really important to read it and you will have a really good moment.