Probably one of the most under-appreciated study tools is the simple scripture index at the back of books. For avid readers this tool provides a memory jog for the book's content and a useful reference for the author's commentary on certain passages. I find this extremely useful when reading and researching certain themes and for finding quotes on subjects from favorite authors. Unfortunately few books include scripture indices and it seems like fewer and fewer publishers (and or authors) are including them.
Scripture Indices generally occur in more theological works, which makes sense as these are most frequently used as reference materials. However, more popular Christian authors and more practical Christian literature often touch on useful, accessible and uniquely communicated interpretations of Scripture and can often be rich mines for practical quotes and persuasive explanations of everyday theology.
In addition to being valuable as a research tool, Scripture indices can often reveal the depth of the biblical basis for the book. Surprised by the Voice of God by Jack Deere is both a practical book, explaining ways that God communicates to ordinary believers in ordinary life, but also apologetic and theological, providing biblical foundations for a sound theology of practical, subjective revelation and the continuance of the manifestations of the Holy Spirit. Because of this, either the author, the publisher, or both felt the need for a thorough Scripture index. The length of the index reveals that the basis of the book is biblical interpretation. Deere cites Scripture frequently and writes from Scripture, so even if one disagrees with the author's conclusions, it must be admitted that his theology is based on Scriptural interpretation.
Similarly, J.I. Packer's practical systematic theology, Knowing God, also includes a thorough Scripture index. This is also a deeply theological work but also a fairly accessible work. Packer's theology holds a lot of weight within evangelical and reformed circles and his interpretations of Scripture are valuable commentary.
Bible software libraries such as logos and libronix often feature filters that pull scripture references into indices and can create a Scripture index for an individual book as well as for an entire digital library. This is an enormous benefit; however, those of us who don't have access to that software must resort to more analog means of documenting scripture references. E-books are fairly easy to create indices for, since a simple search for colons will pull out references to verses (ctrl + f ":").
Paperback books require a more disciplined approach. Many books feature a few blank pages at the back; I often use these pages to record my own Scripture index. When a scripture is cited and commentary or interpretation is given, I take time to flip to the back of the book and record the Scripture reference followed by the page number. I ignore verses mentioned only in passing and focus on verses the author makes a significant comment on. I've been making Scripture indices in the back of my books for over a year, and I have found that it has helped me memorize popular verses, but also remember references better. It's more common for me to use a separate piece of notebook paper as a bookmark now, since it can be irritating to flip back and forth trying to write in the back and keep your place at the same time.
I haven't made a Scripture index for every book that I've read in the last year, but I've been diligent to do so for certain topics that I believe I will be studying for some time or books I will want to go back and glean quotes from. I began the practice because I was approaching a book and an author that I didn't trust, and I wanted to document the biblical foundation (or lack thereof) for the author's doctrine. To my surprise I wound up with five double column pages of scripture references. I walked away from the book impressed by the author's commitment to biblical interpretation and greatly informed about the topic. I did not agree with every interpretation presented, but on the whole, my study found that the author was on solid biblical foundation.
The practice was so fruitful that I've continued it. It has been an aid to memorization, basic Bible knowledge and familiarity with the locations of passages. In addition, it forces me to pause at every mention of Scripture, giving me time to think, "What does this Scripture really mean? Is the author interpreting correctly?" It also makes me aware of the author's foundation. As a whole I would recommend the practice, and I hope to continue and perhaps expand this practice to include every book I read. The Bible is the authority and all Christian literature should lead me into an encounter and re-encounter with Christ, His Word and sound doctrine.