I doubt that the book is affecting Natsu in any way, at least not yet. Natsu wasn't really angered, and the way he said "I'm fired up" most likely means he just got serious. Which is why he most likely activated DF. I mean, both Natsu and Zeref weren't surprised that he achieved it. Zeref looks intrigued, but he doesn't seem surprised at Natsu's power up. So I think it's pretty obvious that Natsu can achieve it at will. I mean, if he managed to do it in Tartaros(granted, it was triggered by emotions back then), I see no problem in him learning to acheive it over the 1 year timeskip. I mean Wendy did it, so why can't Natsu?