Many years ago, after reading quite a number of Christian paperbacks and short books, I decided that my time would be better spent reading more enriching serious Christian literature. I read Calvin's Institutes. My effort was repaid, and while I didn't always follow where Calvin lead, there remained a depth that was encouraging, thought provoking and stimulating of growth.
I have recently opened up a not heavily serious book: Grudem's Systematic Theology, but it is a very solid and well constructed one.
The chapter on prayer was short for the topic, but one of the most useful pieces I've read on prayer. Plenty to ponder upon, and much to encourage. It changed me!
Some people disdain the project of systematic theology in lieu of biblical theology. Both are necessary; systematics develop from the data in the Bible a conspectus of a topic and bring a way of reading the Bible to build understanding of what is taught about a theme. Highly useful, in my view.
I'm looking forward to completing my reading of Grudem and moving on through a few more modern one-volume systematics, before tackling a few multi-volume endeavours. I'm particularly looking forward to Kelly, Jenson, Bavinck and Pannenberg (I just await a low cost set of Pannenberg...tres pricey even at Better World Books).