This probably isn't the "best book" anymore, if it ever was, but
HTML Utopia: Designing Without Tables Using CSS
really made most stuff click with me, and I was mostly reading it.
Because (and this is only about half-way through, the first half is basic junk you've likely seen before) they start with some XHTML, and some designer's image, and show step by step, element by element, how to make each section look like the image. Which is really really nice.
However it's still basic. It gets a wee bit into floats (and via a diagram is the first time I could "see" what floating/clearing meant) and a bit of positioning, but mostly just margins and things, which is how you should position most stuff if you can anyway.
It did not get into float containment (enclosing floats rather than just clearing them), positioning for sprites, or that sort of thing. However once I was done with that book, I was able to pick up most of these particulars on the various tutorial sites pretty well.