You've heard the saying "Don't forget the batteries!" That saying certainly still applies, but if there is an NXT kit for someone on your Christmas/Holiday gift list, I would also encourage you to not forget the instructions! The LEGO NXT kit comes with minimal instructions, so by adding a book or a CD to your gift, you can open up many, many more doors for the recipient and provide for much more fun and enrichment. This is similar to "Don't forget the software" when buying a computer.
And if your special someone already has an NXT set, adding a CD or book can really help re-energize their existing NXT set, so a CD or book gift for them is a great value.
In my experience teaching hundreds of kids robotics over the years, and feedback from their parents, when the the kids are at home working on their own set, the majority of kids need some help with building instructions, and almost all kids need help with the programming. Providing the extra instructions with the gift (or after the gift) can really make the difference between that gift that gets used for a week, or one that gets used and grows with them for years.
For a CD guide with lots of building instructions and programs, take a look at:
50 Fun Projects for your LEGO MINDSTORMS NXT, and for a great selection of books, take a look at this NXT Book List.
By buying one of these products, you are also helping to support the NXT community. I use the money from 50 Fun Projects for your LEGO MINDSTORMS NXT to donate to FIRST LEGO League teams and FLL in general, and for many of the authors of the NXT Books, book revenue allows them to put a dent into their day job to also spend time and resources providing help and information to the NXT community on the internet and in person. Thanks for your support!