Use the online bookstore and see the recommendations for books pertaining to the sermon series 'Blessed Assurance'
Other book recommendations:
The Letters of John - by: John R. Scott
Life in Christ: Studies in 1 John - by: Martyn Lloyd-Jones
Formal Relgion - by: JC Ryle Formal Religion
Heaven on Earth - by: Thomas Brooks
The Letters of John - by: Colin G. Kruse - Pillar Commentary pp.198-ff-the basis of Assurance
Unfailing Love in Uncertain Times (A Journey thru 1 John) - by: Ryan Wicker