I found this book to be pretty concise and useful. WARNING: the author is Christian, and the book gets kind of goofy towards the end (BTW some progressive Christians are really getting into family systems theory, great news). Don't let that scare you away though. I'm a fairly stalwart atheist and it didn't ruin the book for me, so, unless you're Chris Hitchens (and you're not) you aught to be fine. It's still a pretty great primer for family systems theory, well worth it.