Even in 2015, The moshman's book is considered the top-book for
Before this book came out, the best one for SNGs was
Earn $30,000 Per Month Playing Online Poker by Ryan Wiseman.
Having read both of them, i say that
Sit 'n Go Strategy: Expert advice for beating one table poker tournaments is way better than
$30,000... book. It gives many, many examples on many concepts.
Other players also recommend
Secrets of Sit'n'gos by Phil Shaw, but this also came on 2008, and personally i found it a bit difficult book and in no way as easy as the above ones.
So, i believe the moshman's book is still the best out there.
There's also another book (don't know when it came, though), called
Let's play poker by Lee Nelson, specific for SNGs (in collaboration with pokerstars). Pretty good read as well.