By accident I came to this question-discussion only today.
The theme of several answers and comments, that many readings in different styles is best, I'd second, at least up to a point.
I would disagree with all advice to refuse to move forward without "mastery of all details prior"... certainly for nearly all textbooks, and even many higher-level monographs. The reasons is that textbooks currently seem to have the style of belaboring every possible detail, in the name of "rigor", as well as being rather sub-verbal about it. That is, the relative significance of different details/lemmas/whatever is not at all delineated. Since at least 90 percent of details are not at all "dangerous", and not even terribly surprising or illuminating, this results in gross inefficiency. Textbooks are 10 times longer than they need to be, and the critical points are lost in a 10-times-larger mess of fussy details. Terrible.
The only serious approach to avoiding drowning in the faux-rigor fussy details is to make at least one pass through material to see the big points, the higher-level plot-arcs. This lends coherence to the lower-level details. "Hindsight" of a sort.
In particular, "exercises" are an extremely volatile issue. Contemporary textbooks "must" include lots-and-lots of exercises to please publishers and meet other expectations. Thus, one has scant idea of the nature of a given one! Also, one can observe the schism in many texts between the "theoretical" nature of the chapter, and "problem-solving" nature of the exercises, with dearth of prototypes in the chapter itself, to maintain a sort of misguided "purity".
So: distinguishing the relative significance of details, and seeing the larger story-arc, are the most important things to cultivate. Some acquaintance with lower-level details is obviously useful, but the purported "ultimate" significance of low-level details is mostly an artifact of the way mathematics is taught in school.