Simon was living an ordinary life, he had started a band that could win him over girls, he was getting ready for college, when suddenly he was told that he was definitely not ordinary, not even close, and that he isn't who he thinks he is.
This is Simon's story to becoming a Shadowhunter with the help of his very-beautiful-very-bad-ass-shadowhunter girlfriend, and his-now-bad-ass-shadowhunter best friend and her annoying humorous boyfriend, and a warlock who happens to be dating his girlfriends brother.
This dates after the City of Heavenly Fire and before The Dark Artifices.
WARNING: If you haven't read City of Heavenly Fire, be warned, there are a TON of spoilers, basically the whole book is a one big spoiler. Doesn't matter, you wouldn't understand it if you haven't read CoHF at all.