The first sentence is dealing in the present and in the first person. It can either stand by itself, or be the prelude to something else in the present or the future; for example, "It has been a busy weekend, but I am feeling very well rested".
The first sentence deals with the past, and not in the first person. It would not normally stand by itself, but would normally be the prelude to something else in the past; for example, "It had been a busy weekend, and Jack was exhausted".
You haven't actually asked a Second Question,but I would express the sentence as "Finally, I had the time to read the book which you
hadlent tome earlier".
Student or Learner