Amazon.com: Marine Diesel Engines: Maintenance, Troubleshooting, and Repair (9780070096127): Nigel Calder: Books
Marine Diesel Engines by Nigel Calder.
This book is a good one to have. I have a 108 as well. They are an old but good design. A course is a good idea if you don't have much experience with car engines. If you do, then going to diesel is just a matter of understanding the differences. Most of the differences gas and diesel won't even mater when you talk about typical maintainance items.
One thing I would advise, is to make sure that what ever class you take covers small older engine and ideally marine engine issues. I would guess that most of the classes you will find at community collages and such, will cover large, modern truck diesels, because that is where the market is. These engines will actually have very little in common to your 108, outside of the basic function.
One of the things I've noticed over the years is how little information there is available for diesel engine, and the Perkins 108 in particular. This is surprising considering how many of these engines are out there. So for particular questions about a specific issue, you will have to dig a bit.
I have a shop manual that covers the 4.99, 107 and 108. I think it's scanned from a 1973 manual, which is way older than my engine, but the engines didn't change much after that time. If you would like it send me a PM and I'll email it to you. I recommend taking it to kinkos or similar and having it printed and bound, so you can have it at the boat.
Good luck, they are great engines that can take a real beating and will last as long as you keep them repaired.