Once a saint is canonized, we can be assured of their holiness. But the fact that they are saints does not then guarantee that everything that they write or think they experienced is correct theologically.
The Church has said little about Purgatory, other than that it does exist. But whether it is a physical place or a state or what happens there is unclear. So please just keep that in mind as you read books about saints’ visions.
Regarding penance, it is a tradition of the Church to do some form of penance, especially on Fridays. We also should offer each day as an offering in reparation for sin. I recommend that you consider saying the “Morning Offering” each day. It goes like this:
“Oh Jesus, through the Immaculate Heart of Mary, I offer Thee my prayers, works, joys and sufferings of this day for all the intentions of Thy Sacred Heart, in union with the Holy Sacrificce of the Mass throughout the world, in reparation for my sins, for the Holy Souls in purgatory, and for the intentions of the Holy Father. Amen.”
This way your entire day can make reparation for sin. Doing what you normally do, but making an effort to do it well for the love of God and neighbor is also meritorious.
Finally, consider getting a copy of the Book of Indulgences. There are so many ways to receive partial and plenary indulgences that can remit some or all of the punishment due to sins already forgiven.
For more about Indulgences, please see our tract “Primer on Indulgences”