Doctrinal Books More Helpful than Devotional Books?

C. S. Lewis says so, from his' famous introduction to a translation of Athanasius: On the Incarnation:

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.

(HT: Justin Taylor)