Honestly, every single quest in skyrim is well documented in free wikis online.
I use my smartphone's browser if I get stuck in something unsolvable, plus, as patches come out that correct things, the wikis are updated and are more accurate than bound guide books.
That said, I DID buy the deluxe AC3 guidebook, for the litho, map, and the fact that they tried to write it "spoiler free".
As far as spells, potions, smithing and the like, googling is really your best guide to skyrim.
(250 hours +)
While I don't disagree that the walkthroughs that can be found on the internet are well done, the Strategy Guide was written by probably THE top guide author in the business.
With over 100 pages of info before the quest walktroughs,
Walkthroughs that don't hold your hand, and also let you know where quests intersect,
A section on world interactions and encounters and a 200+ Atlas section that details every single location
It is by far the best Strategy Guide I have in my collection. It's actually almost an encyclopedia, rather than a guide.
I know some people don't feel the need to spend the extra money when they can find the same info online, but others like to have an extension of their favorite game. Something they can take with them, sit on the couch and read with their feet up.