Some might argue that all one needs for Bible study is a Bible, and to some degree that is true. However, one must realize that the Bible was written in Hebrew and Greek to people in a particular culture and time. Most people don't know how to read Greek and Hebrew, so they must rely on a translation. While modern translations are quite good, there are always compromises made when translating from any language into another. For instance a word-for-word translation misses Hebraisms and Greek aorist constructions. Many translations provide footnotes for more important passages, which alleviates this to some degree. On the other hand, "dynamic equivalence" translations run the risk of the translators altering it according to their own theology. On the far extreme, there are paraphrases, which are more useful as commentaries than as actual Bibles.
Therefore, there are many useful resources that can be used in Bible study. Some are in the public domain and some are modern. They vary in size and depth, some being intended for the masses while others are oriented to scholars. Commentaries, Bible dictionaries, Interlinears, and lexicons are the most useful to passage studies since they often provide the missing cultural context, and can address the underlying the original language where English translation is often insufficient to convey the full meaning. Bible encyclopedias are useful for topical studies and some historical context. Atlases can provide geographical context, since much of the Bible discusses places that most of us are unfamiliar with. Then there are other reference works which can help with Biblical context, such as chain references and the Treasury of Scripture knowledge, which allows you to find all passages related to each verse of the Bible.
An exhaustive list of all of the available reference works would be prohibitive, but most fall into one of the following general categories.
Bible Translations. It can be helpful to read a passage in several different Bible versions. Slightly different translations of the original text can help reveal a more nuanced understanding, or could indicate where an interlinear Bible would be useful. There are also specialty Bibles, which include commentary or colorize verses by topic, parallel Bibles (which show several versions side by side for comparison), and polyglot Bibles (which shows different languages side by side - such as Greek, Latin, and English).
Bible Paraphrases. Not technically translations, these are versions of the Bible that are restated in (typically) modern idioms. They can help in gaining a better understanding of the Bible, but also run the risk of reinterpreting it through the paraphraser's viewpoint. Examples include the Living Bible, The Message, and Bibles oriented to children.
Interlinear Bibles. These works contain the original language (Greek or Hebrew) with the corresponding English words above or below the original words. In conjunction with a lexicon, this can provide greater insight into the meaning. These are specific to given Bible translations, in terms of the English words and phrases that are used.
Concordances. These are essentially indexes into the Bible. They are specific to a given translation. They can help you find all instances of a word used in the Bible. An example of this is Strong's Concordance. Some concordances are topical in nature - they help you find all references to a topic in a Bible. An example of this is Nave's Bible Dictionary.
Commentaries. These are organized by Book, chapter, and verse. They can range from extended footnotes for a verse (or words in a verse), or they can provide extensive exegesis. They often include cultural context and cross-references to related material. One of the most famous commentaries was written by Matthew Henry. But there are numerous others written by other scholars, including Adam Clarke, Albert Barnes, Luther, etc.
Lexicons. These are essentially dictionaries of original language (Greek and Hebrew) words. Examples of lexicons include Thayer's, Abbott-Smith, and Strong's.
Bible dictionaries. Contains brief articles on biblical terms, people, places, and events. Bible dictionaries are generally more concise, frequently published as single volumes. Examples include: Smith's Bible Dictionary and Unger's Bible Dictionary.
Bible encyclopedia. Bible encyclopedia provides longer, more in-depth articles on a wider range of related subjects, often including topics not explicitly in the Bible. Encyclopedias are typically more comprehensive, often multi-volume works. Examples: International Standard Bible Encyclopedia (ISBE) and Wycliffe Bible Encyclopedia (Two volume set).
Doctrinal. This includes systematic theologies, books that teach or overview specific doctrinal topics, mass appeal books on popular topics from a Biblical perspective, etc. Examples include Calvin's Institutes of the Christian Religion and Protestant Bible Interpretation by Dr. Bernard Ramm.
Christian apologetics. Works that defend the Scriptures and the faith. For instance, "Evidence That Demands a Verdict" by Josh McDowell.
Bible atlases. Works that include maps and descriptions of the geography of the Holy Land.
Devotionals. Works that are intended to be read on a regular basis and contain short devotional messages. For instance, "Morning and Evening" by Charles Spurgeon, which has devotions for the evening and morning for each day of the year. Or "Sparkling Gems in the Greek" which has daily devotions in which a Greek word is examined.
Other. There are several works which don't quite fit in the above categories, such as the Treasury of Scripture Knowledge, which is primarily an exhaustive list of related verses, or books of quotes related to the Bible that can be used in sermons. There are also collections of sermons given by famous preachers, books that provide cultural context, and books which are a combination of some (or all) of the above.
There is an embarrasment of riches available to us in terms of useful study helps. It may be difficult to determine which ones would be best - especially since some of the available works are revisionist or theologically skewed to a particular viewpoint. Some authors are more interested in being "original" than in unbiased historical and scholarly accuracy. So how does one navigate all of this material? First, one should realize that a difference of opinion isn't necessarily a bad thing. Understanding different views of a verse, passage, or doctrine can help you understand the complexities of the subject so that you can make a reasoned and informed decision of your own. We recommend, however, that you avoid material from fringe or cult sources, and those which make everything in the Bible symbolic. If everything is symbolic, you can make the Bible say whatever you want. This is the opposite of Bible study, which has the purpose of understanding the intended meaning of the Bible.
Finally, there are software helps, such as the erasmus Bible app. Bible software provides a library of reference works and a means of easily making use of them. For instance, instead of having to go to the Treasury of Scripture Knowledge for a verse, then look up every related verse in your Bible, you can choose to see related verses in the app, and simply scroll through the verses without the work of having to manually look up each one. Commercial apps tend to provide a curated library of works that are generally useful and reliable. erasmus provides several libraries of different sizes which include reference works which are reliable and treat the Bible with due respect. Even if you use another app, you can look at the list of works included with erasmus and see about obtaining those for your software of choice. With that base of respectable works, you can eventually branch out and try others as you grow in your walk with Christ.
Note: we'd recommend you avoid the Verbum Study App since it is sectarian (Catholic).