The beginner parts were _a_ gap, but they weren't the only gap. In fact, none of the existing books really went very far beyond Monad, with only RWH covering monad transformers.

Having finally got productive using Haskell (on my second attempt at learning it), I can concur, if you're an already seasoned programmer, you need to grok transformers and mtl before you'll feel as useful as you are elsewhere.