You can use the horizontal dimension of the timber as long as you convert the dead load to account for this. Snow load is a projected load so no conversion is necessary.
Concerning your second question, if you are asking for a good design book try "Design of Wood Structures" by Breyer, Fridley and Cobeen. It doesn't concern timber framing directly, but will give you the basics of wood design that can be used in timber framing as well.
As always, nothing is a substitute for a properly trained and licensed engineer, someone I would encourage you to contact should you be looking to put your designs into practice.
Engineering is far more than span tables.