No book has inspired me as much as Catherine of Siena’s The Dialogue. Although the work of Teresa of Avila comes close (I’ve not read the Interior Castle yet, but have read her autobiography and the Way of Perfection - there is a great study guide edition of that available from Inistitue of Carmelite Studies).
Right now I am reading The Practice of the Love of Jesus Christ by Alphonus Liguori (a doctor of the church). It is a very accessible and brilliantly inspiring book, and I wonder why I never heard about it, but have heard so much about books like *An Introduction to the Devout Life *and *The Imitation of Christ, *which are written in a similar vein, but I think that Liguori’s work lends itself more to group discussion. He draws from the work of several saints, some of whom are doctors (including Frances de Sael, John of the Cross, Teresa of Avila, Augustine, and Thomas a Kempis).