Hi Alanabit! Welcome to the world of 8-string obsessives! The mandolin is a wonderfully versatile little instrument, comfortable in a huge range of styles of music; what were you thinking of playing?
The best instructional books I have found are the ones by Simon Mayor, available Here
Another good idea is to join the co-mando discussion board. Lots of good and truly great players drop by, and are very happy to help a beginner along. The people on that list taught me more about the mandolin in two weeks than I'd learned in the previous two years. You can sign up here There is an archive of thousands of tunes here as well, in Tabledit format; this programme shows standard notation, tab, fingering (by using an animated fretboard) and plays the tune in MIDI.
Hope this helps
Tim from Bit on the Side