Which book to become better at photoshop?

[TriggerHappy]

I'm looking for a book that can teach me more advanced techniques in photoshop.

I can do the basics like sharpening, curves etc. but want to work more with things like masking and layers to achieve greater effects so I'm looking for a comprehensive book that can help teach me to use the full power of photoshop (CS5). I'm running mac if that makes a difference for which book you recommend...no use being taught windows hotkeys! So, any suggestions?

[tiler65]

Something like this, Scott Kelby is pretty definitive.

http://www.amazon.co.uk/Photoshop-Digital-Photographers-Voices-Matter/dp/0321703561

[mr_pants]

Not a book exactly (Ok, it's a website), but it might be useful.

http://www.photowalkthrough.com/

[rereg2010]

I had the Dummies guide to photoshop but found it didn't quite do it for me. I now get the advice I need from TP or Google as there seems to be loads of info on t'interweb.