A.
I don't know much about books on the subject, but we've tried a couple of different seats. We like the seat that fits on top of the bigger toilet seat. It makes my son feel like an even bigger boy to sit on the same pot as mom and dad. He can't get on and off it by himself, which is a little drawback, so you'll have to decide if you like that or not.
The other chair we had was a wooden potty chair that my in-laws got at a craft show. Very cute, with a magazine rack on one side and his own toilet paper roll holder on the other, but not at all practical. The seat area is too narrow for a boy to have the pee shield in between his legs so we never used it. Other versions of chairs would probably be easier to work with and more practical, but that's part of the reason why we liked the seat insert on the big toilet.
Hope that helps!
A.