This is a very comprehensive treatise on Prayer by a Doctor of the Church, St Alphonsus Liguori and is a must read for all Catholics. There are so many elements of prayer that are misunderstood or simply overlooked that are essential to our Catholic Faith and especially for Salvation.

I have already published several spiritual works, but of all these works,
I do not consider one more useful than this little book which treats of
prayer as a secure and necessary means of obtaining salvation and all
the graces necessary for it. Were it in my power, I would publish as
many copies of this little work, as there are Christians on earth, and
would give to each a copy, that each might be convinced of the absolute
necessity of prayer for salvation. -St Alphonsus Liguori