I suggest no particular work as a start.The attempt would be futile in the space of my command.But I have two general suggestions of a certain importance.The first is to define the direction and scope of your efforts.
Choose a limited period,or a limited subject,or a single author.Say to yourself:"I will know something about the French Revolution,or the rise of railways,or the works of John Keats."And during a given period,to be settled beforehand,confine yourself to your choice.There is much pleasure to be derived from being a specialist.
The second suggestion is to think as well as to read.I know people who read and read,and for all the good it does them they might just as well cut bread-and-butter.They take to reading as better men take to drink.
They fly through the shires of literature on a motor-car,their sole object being motion.They will tell you how many books they have read in a year.
Unless you give at least forty-five minutes to careful,fatiguing reflection (it is an awful bore at first)upon what you are reading,your ninety minutes of a night are chiefly wasted.This means that your pace will be slow.
Never mind.
Forget the goal;think only of the surrounding country;and after a period,perhaps when you least expect it,you will suddenly find yourself in a lovely town on a hill.