What is the optimal contrast ratio for best readability?
Short answer
Higher contrast increases readability.

Background
In a series of papers under the umbrella "Psychophysics of Reading" (link is to first paper in the series of five) the authors investigate parameters that affect readability in normally sighted folks, as well as people with impaired vision. These studies seem particularly relevant to your question.

The authors investigated reading rates as an outcome measure for readability. In paper V of the series (Legge et al., 2007) they tested the dependence of letter size and contrast (black-and-white) on reading rate. Rates were highest (about 350 words/min) for letters ranging in size from 0.25° to 2° visual angle. Within this range, reading rates proved to be tolerant to contrast reduction. For 1° letters, reading rate decreased by less than a factor of two for a tenfold reduction in contrast. The results were very similar for white-on-black and black-on-white text. Reading rate declined more rapidly for very small (&lt;0.25°) and very large (>2°) letters with lower contrast.

Based on these findings, the intuitive answer to your question is, the larger the contrast, the better. I've never heard of decreasing contrast enhancing reading abilities, barred under some very specific conditions of ultra-low vision, regardless of Google's statements on this. Your first reference clearly supports 'higher contrast is better', without the presence of a sort of optimum.

Reference
- Legge et al, Vis Res (1985); 25(2): 239–52