I can recommend Anne Applebaum's history in addition, of course, to Solzhenitsyn's works:

http://www.amazon.com/Gulag-A-History-Anne-Applebaum/dp/1400...

http://www.amazon.com/Gulag-History-Anne-Applebaum/dp/140003...

http://en.wikipedia.org/wiki/The_Gulag_Archipelago

http://www.amazon.co.uk/Stalin-Court-Simon-Sebag-Montefiore/...

http://www.amazon.co.uk/Whisperers-Private-Life-Stalins-Russ...

I can recommend Gulag: A History by Anne Applebaum:

http://www.amazon.com/Gulag-History-Anne-Applebaum/dp/076790...

The Gulag was created as a terrible economic experiment - that having a large body of "free" slave labour would be good for the Soviet economy.

There were huge, and often pretty pointless, projects built with slave labor and very basic tools - one example being the Baltic - White Sea Canal:

http://en.wikipedia.org/wiki/White_Sea_%E2%80%93_Baltic_Cana...

It's either from:

http://www.simonsebagmontefiore.com/stalincourtredtsar.aspx

or

http://www.amazon.com/Gulag-History-Anne-Applebaum/dp/140003...