Hi!
If you take a look at Ciel's Trivia/Anime section, you will find out that Ciel's transformation into a demon was abandoned starting with BoC. "Why is that so?" is the question that follows. And the answer is: The second half of S1 and the entirety of S2 are not canon. They were anime-only stories and didn't have anything to do with the actual story (aka the manga's story).
Every "Book of" anime adaptation of Kuroshitsuji (Book of Circus/Murder/the Atlantic) follows the happenings of the manga again. And this could only be done if the second half of S1 (and the anime-only elements of the first half like Ash) and the entire S2 were retconned. After all, Ciel is definitely not a demon in the manga, there is no Alois or Claude, there was never an Ash/Angela abomination or a Pluto, Ciel still hasn't found out who attacked the manor, Undertaker being a Grim Reaper was first revealed in the Luxury Liner Arc (which BotA adapted) and he didn't retire but desert...
The second part of S1 and S2 are basically "just there" and don't add anything to Kuroshitsuji's actual story. Its story continues with BoC.