With a pipe in the teeth and a pencil in the hand...

There is a passage in C.S. Lewis’s introduction to St. Athanasius’ On the Incarnation, which I think speaks great wisdom on the subject of devotional material. Note that he is not negating all devotional works, and for the record, neither am I:

“For my own part I tend to find the doctrinal books often more helpful in devotion than the devotional books, and I rather suspect that the same experience may await many others. I believe that many who find that "nothing happens" when they sit down, or kneel down, to a book of devotion, would find that the heart sings unbidden while they are working their way through a tough bit of theology with a pipe in their teeth and a pencil in their hand.”

I personally have found the same to be true. Most recently, as I’ve been preaching through Matthew and working my way through the Sermon on the Mount, I’ve found my heart bursting at Calvin’s exposition of the Lord’s Prayer in the Institutes. There is much light but it is accompanied by warmth; it is red meat but seasoned well and with a gracious aroma. John Stott's expositional commentary on the Sermon on the Mount is also great. 

In fairness, I have found some very good devotional books as well, but they are typically the ones which are based on exposition of biblical passages or on pondering the implications of various doctrines or attributes of God taught in Scripture or events recorded in Scripture. Charles Spurgeon’s Morning and Evening and John Piper’s A Godward Life, immediately come to mind as good devotionals. However, if I were to compare, I’d say that both Spurgeon’s and Piper’s works of theology or exposition have done more than their works of devotion to fire the faith of the saints.

Some works which have caused my heart to sing and which I would highly recommend:  

J.I. Packer's Knowing God, and his Growing in Christ; John Piper's Desiring God, and his Future Grace; R.C. Sproul's The Holiness of God, and his Chosen by God; A.W. Tozer's The Knowledge of the Holy; Dietrich Bonhoeffer's The Cost of Discipleship; Jerry Bridges' The Pursuit of Holiness. 

Of course, none of these works would be considered "a tough bit of theology", but are rather all pretty accessible.  However, each of them deals with bits of tough theology and will stretch any honest believer's mind and soul.