To give yet another example, I've been slowly trying to teach myself GR, and while I love the approach and the rigor of Wald's General Relativity, it was too hard for me to follow on its own terms. I found that Schutz's A First Course in General Relativity provides both the insight and better grounding in some of the necessary math (tensor analysis, getting used to Einstein's summation convention, using the metric to flip indices around) through gentler approach and richer examples. Having studied Schutz for some time, I feel (almost) ready to come back to Wald now.