This is probably the best all-around book to give to someone who is struggling with “assurance of salvation”:

Mike McKinley. Am I Really a Christian? The Most Important Question You’re Not Asking. Wheaton: Crossway, 2011.

(See Brian Hedges’s review and my previous blog post on Mike’s first book, Church Planting Is for Wimps.)

The book’s website has several videos. In this one Mike answers the question, “Can someone believe the gospel but not really be repentant?”