All of these resources have some information related to the Tablets of Bahá'u'lláh or their history. While this list is not meant to be exhaustive and doubtless omits some people's personal favorites, it does include the books one might find most essential. I did not include generalist texts, such as Hatcher/Martin's The Bahá'í Faith, Peter Smith's The Babi and Bahá'í Religions, or Moojan Momen's A Short Introduction to the Bahá'í Faith. Lists of these generalist intro texts can be found in the Resource Guide, cited below. Listed alphabetically, in two sections: Secondary Books and Bibliographies and Dictionaries.
The Covenant of Bahá'u'lláh. Adib Taherzadeh. Oxford: George Ronald, 1992. 473 pages. This lengthy book can be used almost as a companion volume to Taherzadeh's Revelation of Bahá'u'lláh series. Though topically only about the covenant, the length and amount of detail included renders the book widely useful--it contains much historical information. It is followed by its sequel, Child of the Covenant, which focuses on the Will and Testament of Abdu'l-Bahá.
God Passes By. Shoghi Effendi. This book needs no introduction; I just include it for the sake of completeness. If you don't have a copy, get one. The most recent hardcopy edition, partially underwritten by the Bahá'ís of the United States (I believe), costs only US$13.00.
Miracles and Metaphors. Mirza Abu'l-Fadl Gulpaygani, trans. Juan Cole. Los Angeles: Kalimat Press, 1981. 211 pages. While all of Mirza Abu'l-Fadl's works are good, this particular book not only sheds light on Christianity and Islam from a Bahá'í perspective, but also contains much insightful commentary and philosophy about the nature of revelation and symbolism in the Writings and Their interpretation.
The Ocean of His Words. John Hatcher. Oxford: George Ronald, 1997. 388 pages. This is one of the longest extended studies of the written revelation of Bahá'u'lláh, and one of the best ones that examine in any depth the literary qualities of the Writings.
A Resource Guide for the Scholarly Study of the Bahá'í Faith. Robert Stockman and Jonah Winters. Wilmette: Research Office of the Bahá'í National Center, 1997, 229 pages. This book was written as a comprehensive resource tool for educational and academic approaches to the Bahá'í Faith. It contains topical bibliographies sorted by subject (e.g. "Bahá'í History," "Holy Places, Bahá'í," "Mysticism," "Psychology," "Religious Dialogues with Islam," "Unity in Diversity," etc.). It also contains sample curricula for Bahá'í college courses, a Bahá'í glossary, lists of Bahá'í publishers' addresses, lists of Bahá'í videos, etc. It is available online, at http://bahai-library.com/books/rg. See also its overview of online materials and the topical guide to scholarship.
The Revelation of Bahá'u'lláh, vols. 1-4. Adib Taherzadeh. Oxford: George Ronald, 1974-1987. These detailed books are the standard guide to the content and context of the writings of Bahá'u'lláh. Volume 1 covers the Baghdad period, 1853-63; vol. 2 covers the Constantinople/Adrianople period, 1863-68; vol. 3 is the early Akka period, 1868-77; vol. 4 is the late Akka/Mazra'ih/Bahji period, 1877-92. These four books, along with Balyuzi's Bahá'u'lláh: King of Glory, provide the most complete biographies of Bahá'u'lláh. While written confessionally, they are very well-researched and scholastically useful.
Sacred Acts, Sacred Space, Sacred Time (Bahá'í Studies, volume I). John Walbridge. Oxford: George Ronald, 1996. 322 pages. This is a very useful collection of articles on the Faith and its history. It includes sections on, for example, Bahá'í Law, pilgrimage, many Bahá'í Writings, mysticism, the Bahá'í calendar and holy days, etc. It is useful partly because the author has often found information lacking elsewhere, such as more exact dates of revelation for certain Tablets and the context in which They were revealed. These essays were originally written for the Short Encyclopedia of the Bahá'í Faith but were published separately in advance. Some of its sections are online at the encyclopedia page.
Scripture and Revelation (Bahá'í Studies, volume III). Moojan Momen, ed. Oxford: George Ronald, 1997. 369 pages. This is a collection of talks given to the Haj Mehdi Arjmand colloquia, rewritten and edited as essays. Included are useful papers such as Robert Stockman's "Revelation, Interpretation, and Elucidation in the Bahá'í Writings," Ross Woodman's "The Inner Dimensions of Revelation," and essays on mythology, love in Bahá'í mysticism, religious exclusivism, and Zoroastrianism.
Studies in the Babi and Bahá'í Religions. Anthony Lee, general ed. Los Angeles: Kalimat Press, 1982-. There are currently 10 volumes:
A Basic Bahá'í Chronology. Glenn Cameron with Wendi Momen. Oxford: George Ronald, 1996. 540 pages. Though somewhat pricey at US$29.95, this exhaustive chronology is invaluable when it comes to looking up dates. The authors scoured every significant historical or biographical book on the Babi and Bahá'í Faiths to extract or extrapolate the dates for all notable events, and included a brief bibliography for further reading after each entry (marked by abbreviations, e.g. "bkg 342").
A Basic Bahá'í Dictionary. Wendi Momen. Oxford: George Ronald, 1989. 261 pages. This is the only relatively detailed dictionary of the Bahá'í Faith, and as such is very useful. It can seem fairly superficial, since 261 pages is barely enough to offer even a fraction of the entries necessary for a Bahá'í dictionary and it is partly superseded by Peter Smith's Concise Encyclopedia of the Bahá'í Faith (see below) and later the eventual appearance of the Short Encyclopedia of the Bahá'í Faith.
The Bahá'í Faith: A Historical Bibliography. Joel Bjorling. New York: Garland Publishing, 1985. 168 pages. This bibliography, written by a non-Bahá'í, is not very good but must be listed for the sake of completeness. It is lengthy and fairly comprehensive, but it contains a few errors and gives too much space to listing works by covenant breakers. If your local college library has a copy, however, it could make a useful supplement.
Bahá'í Glossary. Marieh Gail. Wilmette: Bahá'í Publishing Trust, first printing 1955. 56 pages. This is, to the best of my knowledge, the only lengthy glossary of Bahá'í terms. Includes pronunciation guide. This is currently out of print, but if you ask around your local community you might find someone with a copy.
Bibliography of English-Language Works on the Babi and Bahá'í Faiths 1844-1985. William P. Collins. Oxford: George Ronald, 1990. 521 pages. This book, along with the two Basic Bahá'í Dictionary and the Basic Bahá'í Chronology listed above, is one of the indispensible additions to any academic Bahá'í library. It is the most complete biblography available, and organizes works on the Faith by title, author, and subject. As the Library Director at the Bahá'í World Centre from 1977 to 1990, Collins is perhaps the leading expert on Bahá'í bibliography.
A Concise Encyclopedia of the Bahá'í Faith. Peter Smith. Oxford: Oneworld Publications, 2000. 396 pages. The most complete encyclopedic dictionary of Bahá'í terms, places, and figures.
Lights of Guidance: A Bahá'í Reference File. Helen Bassett Hornby, comp. New Delhi: Bahá'í Publishing Trust, first edition 1983, fourth edition 1996(?). This is a standard Bahá'í resource which I'm sure every Bahá'í is familiar with, but I list it to be complete.