The storyline chronologically that you should follow:
1) Black Butler (season 1 episodes 1-15)
2) Book of Circus
3) Book of Murder (parts 1&2)
4) Book of the Atlantic
5) Black Butler: Ciel in Wonderland part 1&2
6) Black Butler: Welcome to Phantomhives
7) The Threads of the Spider
8) The Story of Will the Reaper
9) Public School Arch
10) Black Butler (season 1 episodes16-24)
11) Making of Black Butler II
12) Black Butler II (season 2)
The reason why Black Butler season 1 is split in half is because there are extra stroies told in different episodes outside of that season as somewhat standalones. This includes Book of Circus, Book of Murder, and Book of the Atlantic. These are all based in the original manga. The back half of Black Butler season 1 and all of Black Butler season 2 are original and not based off of the manga which means its not canon.
