C. H. Spurgeon 
Around the Wicket Gate / or, a friendly talk with seekers concerning faith in the / Lord Jesus Christ [PDF ebook] 

Charles Spurgeon draws from John Bunyan's Pilgrim's Progress to reach those who have not yet entered the "wicket gate". The free gift of eternal life is available to all, including those in the dangerous position of being almost a Christian.
A great book to share with unbelievers and to read as a believer to help you share your faith. The great Spurgeon's treatise on the basics of the gospel. A short read laying out the gospel as few can. This little book has been a powerful conduit for the gospel in the history of the church.
Contents
Awakening – Jesus only – Faith in the person of the Lord Jesus – Faith very simple – Fearing to believe – Difficulty in the way of believing – A helpful survey of Christ's work – A real hindrance to faith – On raising questions – Without faith no salvation – To those who have believed.