This afternoon I gave a talk at the cake meeting, "Empirical valence bond models for hydrogen bonding and proton transfer". The notes are here.
The figure below taken from a Chemical Reviews is interesting. It shows how for a
O...H-O system the correlation been O-H bond length and the O..H bond length has a natural explanation in terms of bond order conservation. To me this shows that hydrogen bonds are essentially a resonance between a covalent bond and a charge transfer excitation, as proposed by Coulson. i.e. the wavefunction is a superposition of A-H... B and A-...+H-B.