It's not that one needs the Fire Book [3]. Ming simply just wanted to know why the GMs decided to leave it out of the list of slottable items, considering the other 3 books are able to have slots. Which was answered in the topic. The Book of Blazing Sun [3] shares the same Item ID as the Tome of Ymir SQI. Which means if the GMs currently implemented the Book of Blazing Sun to be slotted, you would get a Tome (not the actual Tome, but the Fire Book's script would be the same as the Tome). This is the same issue as what happened with Drill Katars, they had the same ID as Twin Fang, so Drill Katars were in essence a Twin Fang.