Board Thread:Questions and Answers/@comment-31690120-20170407171522/@comment-175.158.225.5-20170407200210

SS2 should be read before starting volume 17 just because it was released first. Since you are in this deep already, just read the sidestories, most importantly Norse Mythology SS, before beginning with the NT novels. Many of the series occur in a vague time frame where Touma is already wearing his winter uniform but has yet to leave the city and there is no indication that the events of NT has started, though some sidestories are set earlier than that

Indeed, Road to Endymion is set prior to volume 9. Necessarius Entrance Exam is set prior to volume 11, Norse Mythology SS's timeline is all over the place, Jihanki no Fanfare is set after Railgun season 1,  etc.,

Just take it easy and finish reading the main novels of the "old testament" first then you can start reading the sidestories.