You said learn Unix... So I assume you're looking for a more Unix-style linux disrto? If so, go with slackware. If you just want to learn linux, try Mandrake.
If you want to get even closer to Unix, try out BSD. There's 3 different kinds, FreeBSD, OpenBSD, and NetBSD. FreeBSD has a ton of ports (linux programs that have been ported over to work with FreeBSD), while OpenBSD is supposedly the most secure OS out there. I don't know anything about NetBSD, unfortunately.
If you're really serious about it, check out the O'Reilly line of Linux books.