James Allen wrote it in 1903 and it has not aged a day.
The argument is simple: your thoughts shape your character, your character shapes your circumstances, and a mind left untended will grow weeds the same way any unattended ground does.
Purpose is what turns thought into direction. Discipline is what turns direction into a life.
If you read one short book this year on how you got where you are, read that one. It takes an afternoon and it will cost you some comfortable excuses.