Loki
As a rule, don't trust the infinity symbol on lenses. It is rarely accurate. Also if using a tripod only, you will not have any tracking ability so you are limited to relatively short exposures, unless you are aiming for star trails anyway!!
This book
http://www.bintel.com.au/Accessories...oductview.aspx is a not bad introduction to using a DSLR for astro. It is getting a little dated and the equipment it mentions are all no longer around, but the principles in it are still relevant. A quick search on the net will reveal more up to date books I am sure.
Malcolm