The author seems to like and remember the books-

Comprehension:

For my own part, I have ever gained the most profit, and the most pleasure also, from the books which have made me think the most; and, when the difficulties have once been overcome, these are the books which have stuck the deepest root, not only in my memory and understanding but likewise in my affections. For this point too should be taken into account. We won’t think slightly of that which it costs us a slight effort to win. The more a book makes us think over, the more satisfaction we get from that. When a maiden is too forward, her admirer deems it  time to draw back.

If you would fertilise the mind, the plough must be driven over and through it. The gliding of wheels is easier and faster, but only makes it harder and more barren. Above all, in the present age of light reading, that is, of reading hastily, thoughtlessly, indiscriminately, unfruitfully, when most books are forgotten as soon as they are finished, and very many sooner, it is well if something heavier is cast now and then into the midst of the literary public. This may scare and repel the weak; it will rouse and attract the stronger and increase their strength by making them exert it. In the sweat of the brow, it is the mind as well as the body to eat its bread.