It was released to celebrate the 15th year anniversary. I don't think they had the book done way back when if that's what you meant by your question given eiwa's statement on the credits page, but I'm sure they've considered making her a book for a long while since they apparently gave him a faux book cover at one point according to his twitter. Aside from that there are some formatting differences when compared to the older books, mainly on the cover, that definitely make me think that it's not something they've just had sitting around all these years.