David Coderre is perhaps the best known ACL user in the world.  He has recenlty re-released his three books through Wiley and Sons.  The first book, Fraud Analysis Techniques using ACL is in my opinion one of the best investments an ACL user can make towards learning ACL.  Dave provides several scripts that can be used to help identify fraud, but the value of the book comes in Dave's explaining how the scripts work.  If you sit down and study these scripts, your script writing skills will improve dramatically.  I highly recommend that book or any of the books Dave has written.  (For those who are not familiar with the ACL user forum, Dave was the original SuperUser and has helped establish a culture on the forums that is open to new users asking questions---often the same questions that have been dealt with over and over again.