From Boswell’s Life of Samuel Johnson:
The particular course of his reading while at Oxford, and during the time of vacation which he passed at home, cannot be traced. Enough has been said of his irregular mode of study. He told me that from his earliest years he loved to read poetry, but hardly ever read any poem to an end; that he read Shakspeare at a period so early, that the speech of the ghost in Hamlet terrified him when he was alone; that Horace’s Odes were the compositions in which he took most delight, and it was long before he liked his Epistles and Satires. He told me what he read SOLIDLY at Oxford was Greek; not the Grecian historians, but Homer and Euripides, and now and then a little Epigram; that the study of which he was the most fond was Metaphysicks, but he had not read much, even in that way. I always thought that he did himself injustice in his account of what he had read, and that he must have been speaking with reference to the vast portion of study which is possible, and to which a few scholars in the whole history of literature have attained; for when I once asked him whether a person, whose name I have now forgotten, studied hard, he answered ‘No, Sir; I do not believe he studied hard. I never knew a man who studied hard. I conclude, indeed, from the effects, that some men have studied hard, as Bentley and Clarke.’ Trying him by that criterion upon which he formed his judgement of others, we may be absolutely certain, both from his writings and his conversation, that his reading was very extensive. Dr. Adam Smith, than whom few were better judges on this subject, once observed to me that ‘Johnson knew more books than any man alive.’ He had a peculiar facility in seizing at once what was valuable in any book, without submitting to the labour of perusing it from beginning to end. He had, from the irritability of his constitution, at all times, an impatience and hurry when he either read or wrote. A certain apprehension, arising from novelty, made him write his first exercise at College twice over; but he never took that trouble with any other composition; and we shall see that his most excellent works were struck off at a heat, with rapid exertion.