This is a great book. One weakness of the Reformed view of assurance is that it doesn't seem to have an objective basis. Beeke argues that God's promises are the objective basis; however, these promises must be appropriated by subjective faith, which seems to undermine objectivity.
Beeke's chapter on evidences of grace helps make up for this weakness. Since one must only see evidence of one of many marks of grace to ground assurance, assurance can be found without inordinate introspection.
The rest of the book features Beeke's usual biblical and experiential teaching. He helps his reader to practically cultivate and renew assurance. His conclusion is a wonderful and urgent call for sinners to come to Christ.
Finally, although Beek does cover the public means of grace as a means to assurance, it all seems very individualistic. I would love to see a chapter on how the church aids believers with assurance: confession of sin, church discipline, etc.