Calder's book is good. So is Don Casey's This Old Boat- a good section on dry/wet exhaust with water-lift mufflers.
I just replaced the dry exhaust riser and wet stack on my A4 with a few specialty parts from Moyer Marine ( also a good souce of info), and about $40.00 of pipe from Home Depot. Pretty straightfoward ( except for contortions in the engine room!) Did it over a few weekends.