On my birthday in 1985 Linda gave me A Guide to Prayer for Ministers and Other Servants. I used it for two years. It became my favorite devotional book, of all time. I required it for students in some of my seminary classes.
It's thematic, cross cultural, biblical, and meditative. If you want to go deep, this will take you there. It has proved to be a good guide for me in life. A month ago I took it off my bookshelf, and am doing it for another year.