At seminary, the professors rightly encourage the students to have a good devotional life. In addition the professors have a seemingly unending list of book recommendations. With these two things in mind, I would like to give my own recommendation of two books that have greatly aided me in my own devotional life.
First I strongly recommend to everyone a good study Bible. A good study Bible will provide notes that help the reader to understand difficult phrases or verses. In addition they often call attention to important passages of Scripture and provide a brief explanation of them. Furthermore a good study Bible includes a sort of meditation at the end of each chapter. Thus, by using a study Bible for devotions one is able to read with the more understanding.
For myself, this has been especially helpful as I just finished reading through the Old Testament prophets. I would guess that most of us struggle to understand the sense behind some of the Old Testament prophecies. I have found that by reading through them with the aid of a good study Bible I was able to better understand some of the difficult language found within them.
While there are number of study Bibles out there, I personally recommend the Reformation Heritage KJV Study Bible. The reason I recommended this one: it explains the language of the King James Version from a Reformed perspective. In addition it has many other study tools and helps. And as an added bonus, it also contains the Three Forms of Unity and the Westminster Standards (that is the Confession of Faith, Larger Catechism, and Shorter Catechism) in the back.
In addition to a good study Bible, I recommend Survey of the Bible by William Hendriksen. This book contains a concise summary of Bible history. In addition for each book of the Bible it provides the reader with a brief summary of the historical setting, or context, and an outline of the book as well. By reading and studying Hendrickson at the onset of each new book that you come to in your devotions, you will have better idea of the purpose and theme for each book. In addition, the outlines help readers understand the progression of thought throughout each book of the Bible. Again this is helpful because it facilitates reading with understanding. This holds true especially for reading through sections of the Bible such as the Prophets.