
Volume 1 (Set between volumes 13 and 14) A series of short stories set before the events of volume fourteen including how Team GROUP was formed. Volume 2 (Set between volumes 16 and 17) A series of short stories including two separate meetings between Touma's and Mikoto's parents, two tales involving two of the Level 5s of Academy City, and other funny stories involving some of the characters. (Source: Wikipedia) Note: Volume 1 contains a prologue and epilogue.
Toaru Majutsu no Index SS is a Japanese novel of 172,159 characters, with 10,275 unique words and 1,862 different kanji. Its difficulty rating on Jiten is hard, 3.6 out of 5. The full vocabulary list ranks all 10,275 words by how often they appear, and you can study them on Jiten or download them as an Anki deck.
Here are some of the words that appear most often in this media, as well as a sample of its rarer vocabulary.