As I'm preaching through Titus I have occasion to use any of dozens of books. One lesser-known commentary on the Pastorals is by Patrick Fairbairn, better known for his works on prophecy and typology.
The title for my series is Titus: Living Sound Doctrine. It's my best shot at capturing the constant back and forth interweaving of the two themes in Titus, showing the interrelatedness of sound doctrine and sound living.
All that to introduce Fairbairn's apposite remark on Titus 2:1.
Christianity is primarily, indeed, a doctrine, but only that it may be in the true sense a life; and the two can never be kept apart from each other in the public teaching of the church without imminent peril to both.
[Fairbairn, P. (1874). The Pastoral Epistles: The Translation with Introduction, Expository Notes, and Dissertations (270). Edinburgh: T&T Clark.]