I read only the first two books, which are his ethical exhortations, which were great; very challenging and relevant, that same tone all good meditative authors seem to have. Some highlights:

It is a great thing to live in obedience, to be under a superior, and not to be our own judges.
It is much safer to obey than to govern.
Many live under obedience out of necessity rather than out of love. Those who do are discontented and easily distressed. They will never obtain peace of mind until they willingly and heartily put themselves under obedience for the love of God.

It may also happen that even though one's opinion may be good, refusal to yield to others when reason or a special cause requires it is a sign of pride and stubbornness.

If every year we would root out one vice, we would soon become perfect people.

We are eager to have others perfect, but yet we do not correct our own faults... The great freedom that others have displeases us, and yet we will not have our own desires restricted.

The fuel that feeds the fire of hell is sin.

If things were well with you and you were purified from sin, all things would work out to your good and your spiritual advancement.
But many things displease and often trouble you because you are not yet perfectly dead to yourself or separated from all worldly things.
Nothing so defiles and entangles the heart of a person as impure love for others.

You know well how to excuse and color your own deeds, but you are not willing to accept the excuses of others.
It would more just for you to accuse yourself and excuse others.
If you want to be endured, learn to endure others.

Jesus now has many who love His heavenly kingdom, but few who bear His Cross.
He has many who desire comfort, but few who desire tribulation.
He finds many companions at His table, but few who fast often.
All desire to rejoice with Him, but few are willing to suffer anything for Him--or with Him.
Many follow Jesus into the breaking of bread, but few to the drinking of the cup of His suffering.
Many revere His miracles, but few follow the humiliation of His Cross.
Many love Jesus, so long as adversities do not happen to them.