Hi! Honestly, if you know Ruby, you really don’t need my book. It’s not even really about Ruby; it’s about programming, just using Ruby as the base language.
If the newest Pickaxe is shorter than it used to be, it’s almost certainly because they used to include the standard library reference at the end. I’m guessing that kept growing to the point where it no longer made sense to include all of it. Also, the standard library reference is online, so you really don’t need a printed copy of it. (I don’t actually have the latest Pickaxe, so the above is just a guess.)
Go for the Pickaxe!