Playing in Photoshop

I'm ready to sit down and learn how to use Photoshop.  With a book.  Very unlike me.  Everything I've learned up until now has been through trial and error or by looking up something specific online and learning how to do it.  I have a working knowledge of Photoshop and that's worked well for me up until now but I'm ready to try something new.   I don't want to be slowed down by my lack of knowledge.   Can anyone recommend a good book or online course?