The understanding and skills from the LAB book are still relevant, as Richard saiid. I use the techniques much less now as CS4 or 5 offers so many new ways of dealing with image editing. Some of my images never leave Lightroom3. But understanding what each of the 10 color channels looks like can contribute to how I edit an image.
If you are considering upgrading to CS5, you should also consider upgrading to Lightroom 3 at the same time. Through Adobe, the upgrades were $199 for CS5 and $99 for lR3. I was pleasantly surprised to find that when I got to checkout, the final price for both was $268. Not a monster amount of savings but every little bit helps.
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