Yes, by all means he/she should buy a Redbook http://www.amazon.com/Guide-Book-United-States-Coins/dp/0794822673
It depends on what you call a key date. As noted earlier, the 1895-O is the "key", but there are also some other dates that can be very expensive even in lower grades, like the 1901-S and 1903-S. If you get a Redbook, you can get an idea of what the key dates are, not only for Barber dimes, but for other series as well.
I don't have my Dansco with me right now...but I don't remember any doubled die slots in it. I think it is just the date/MM set.