Never heard of them. You should still be able to get the factory service manual from any HD dealer.

You will probably want to get the "parts" and "electrical" manuals too. Yes they are expensive, but they are really good.

I don't have first hand knowledge. I've heard the printed manuals are not great but that their online versions are good.

I've generally preferred buying the manufacturer's manual even though they're relatively expensive. Honda's manuals and their Common Service manual in particular were excellent. KTM's is just okay, disappointing after coming from Honda's and BMW's manuals, at least it gets the wiring diagram and torque specs right.

I've had mixed experience with Haynes and Clymer. They're better than nothing but the photos are not so good, they rarely cover differences between years, and make a lot of assumptions.