I highly recommend the Book “Inside PixInsight” which was published in September 2016 and is part of the Patrick Moore Practical Astronomy series. Full disclosure: I was the Technical Reviewer/Editor for this book. You can get it from Springer at http://www.springer.com/us/book/9783319256801, and it’s also available at amazon or through the author directly at http://www.ip4ap.com/books/inside-pixinsight-by-warren-keller (with a personal inscription if you ask).