Interpretation of coefficient of determination (R^2)

I calculatet R^2 via the lm function in R. I examined how differences in my quantitative trait can be explained by differences in the diplotype effect (calculated via ASReml). It resulted in 0.58 for R^2. Or in case of the multiple linear regression (Diplotype effect, age, sex, body weight and height) 0.77. I would like to ask how to interpret these values in my discussion. How good are these values and how much i can trust them. Please advice me some good literature.
Thanks in advance