by Rachel Seigel --- Last week during a Twitter chat, a particular question caught my attention. The participant was wondering if books in a series have to be read in sequence in order for the series to be good. This lead me to consider what does make a good series, and while there are several factors that play into it, for me, the simple answer is that any series that can sustain my interest over several books, and keep me invested in the characters and the world is a good series. The next question then, is how is this accomplished? What are the key ingredients to creating a series that will keep a reader with you over 3, 4, or even 7 books? When asked what has made Sherlock Holmes one of the most successful and best-loved detectives of all time, Anthony Horowitz (who is the author of the Sherlock Holmes Adventure