Unspeakable Comfort

This book explains the Reformed doctrines which are often referred to buy the acronym TULIP:

Total Depravity

Unconditional Election

Limited Atonement

Invincible Grace

Perseverance of the Saints

These doctrines were defended by the Synod of Dort in 1618-19.  Since their basis is in the Holy Scripture the importance of these teachings is timeless.