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by Craig Anderton
is often cited as a good starting-point. I get it's for v.3, but the basics still remain the same.
There are also a few good videos to be found by searching on Youtube, as well as some good ones you have to pay for from the likes of [Only registered and activated users can see links. Click here to register]
, who offer a 30 day "All-Access Pass" for $15. This lets you watch any of the over 1400 hours of videos they host, which includes some on Sonar.
That said, if you have $20 handy, you could grab both, as the book I mention above can be got "used" on Amazon for as little as $2.96 + $3.99 shipping.
Best of luck.