I have just completed reading the book, Forgiveness -- God's Master Key (Pray the most powerful prayer on earth) by Peter Horrobin of Ellel Ministries, published by Sovereign World, ISBN 978 1 85240 520 1
It is a challenging and thought provoking book which presents a water tight case that we should forgive unconditionally as Yahooshua {Jesus} did on the stake {cross}.
I strongly recommend it.