The Imitation of Christ by Thomas A Kempis
While it was written for monks and you must keep that in mind with some parts, it has some amazing advice like "do not think too highly of what you know, for there is always someone who will know more than you, instead be humble."
The book is very much in vein of "I count all things lost but knowing Christ." (Philippians 3:8)
It radically effected my personal faith walk, at a crucial moment in my life. That said, it is part of Roman Catholic Devotio Moderna Movement, that Martin Luther was apart of and caste off later in his Reforms. It can get into too much Asceticism, which The Apostle Paul says is unprofitable (see Colossians 2:23, ESV).
The key is to read it for some of meaty nuggets, and spit out the bones.