Can anyone suggest a good reference book for wire selection / ampacity for DC current?
RE: Ampacity in DC wiring-Any first year 'Introduction To DC Fundamentals' textbook should have all the info you need. Mine was 'Delmar's Standard Textbook of Electricity (Third Edition)', FWIW.
At what voltage and with what load?
Amps is amps as far as wire is concerned. 12 awg THHN carries 20 amps at any AC or DC voltage-for the first ten feet, anyway. The lower the voltage, the higher the voltage drop over distance (worse with DC then AC).
Are you running photovoltaic panel wires from roof down to batteries/inverters? Try to keep power supply as close as practicable to load.
Low voltage DC suffers much more voltage drop then AC. In DC, RETURN path is counted in voltage drop at any voltage. 50 feet of 'hot' from supply to load and 50 ft. of return back to power source equals 100 ft. when figuring wire size relative to voltage drop.
For example, if you need 10 awg for a 40 ft total (20 ft from supply to load and back again) run, you'll need 4 awg for 80 ft (40 ft supply to load).
Then (if we're talking motors) there's the type of load to consider. Dig out your textbook from trade school, it's trigonometry time. That's why my response was-DC used for what, at what voltage?