I've always tuned my Sobell to dD a aa, as many tunes start 'so - do' as Bob suggests. I've never seen the Mel Bay book, though I would normally expect their tutors to give good advice.

Mine has five strings, and the 'bass' string is doubled to the octave, giving it an unusually rich sound. Pete Coe has one the same as mine, but I've never really taken any notice how he has it strung.

The 'bridge nut' isn't in a slot, but is moveable, rather like on a banjo but made from a flat piece of bone with a raised ridge, to allow for differences in strings, which seems a good idea to me!