I don't know if it's just me, but I always find punctuating sentences like this problematic. (a) seems logically correct to me but looks fussy. The style of (b) is no doubt common, but is offputting to me since the commas seem to set off "but having read it over such a long period of time" as a phrase.
A is "technically" correct but looks fussy. (Perfect word choice.) B is not (technically) logically correct, but easier on the eye and doesn't actually lead to confusion about the intended meaning. C ends up looking like someone dumped punctuation on the page.
Let's not forget that the primary role of punctuation is to add clarity