Wow, this is a really far-reaching question, as it covers the whole spectrum both of history and of studying, not to mention the academic level of the student. Some things would be more helpful for certain areas of study or to students of a certain grade, but these two things are helpful tools for students of history.
The best resource for anyone who is studying history is original documents if they are available. In short, this means than instead of reading a book about the Constitution, read the actual Constitution. Rather than reading an article about the Cuban Missile Crisis, find an interview with one of the parties who were actually involved. You undoubtedly own some original documents, such as a birth certificate, a driver's license, and medical and school records. Consider what someone could know about you and presumably other students of this generation and place if they had these documents to examine one day.
These kinds of sources are also called primary sources, and most major universities have descriptions of them as opposed to secondary sources (like the book on the Constitution and the article about the missile crisis). Good primary sources include the following:
- ORIGINAL DOCUMENTS (excerpts or translations acceptable): Diaries, speeches, manuscripts, letters, interviews, news film footage, autobiographies, official records
- CREATIVE WORKS: Poetry, drama, novels, music, art
- RELICS OR ARTIFACTS: Pottery, furniture, clothing, buildings.
Anything that was written, spoken, photographed, or created at the time of the historical moment or era is generally more valuable than anything that was written after the fact. Contemporary sources were "there," so to speak, so they carry more validity.
The U.S. Library of Congress has a wealth of information about all kinds of things, and universities often contain collections of original documents which can be accessed online. The eduplace site linked below is one place to search for primary sources on the internet, though there are many other places to find them, as well.
Secondary sources are works that interpret or analyze those original sources. While they may contain some of the same information as the original documents, they are filtered through the biases, experiences, and worldviews of the writers. There is an inherent distortion simply because of the point of view of each author, and of course this is why two writings by equally qualified people can come up with completely different conclusions (think global warming--I mean climate distortion). Secondary sources have value, of course, but they are not as pure as original documents.
Looking at original documents has the added benefit of making history more fun and accessible. Reading a textbook account of the battle at Little Big Horn is not as exciting as reading about it from someone who was there, for example. Looking at public opinion polls or examining ration coupons from World War II add a new depth of meaning to those events. There is still room for interpretation and discussion, of Course, but they are your opinions and insights based on contemporary data.
Perhaps you were hoping for a book title or some other type of study tool, but really, every one of those is either a primary or a secondary source. If you haven't done so already, try looking for some original documents on your favorite historical event or era (Civil War, Trail of Tears, Boston Tea Party) and see what you can find. You'll make some intriguing discoveries.
I would add on that the importance of primary sources goes back to the idea of putting history back in context. Anyone can memorize a date out of a history book, but to truly understand the facts of history, one must have knowledge of the culture and society in which the event occurred.
For example, the Russo-Japanese War lasted from 1904 to 1905 between the Russian and Japanese Empires over the control of Manchuria and Korea. A simple internet search will tell you that Japan won the war and that Manchuria and Korea were occupied and colonized not soon after. However, when you learn the context of the war, you will learn that Japan's desire to become an empire stemmed from wanting to catch up to the industrialized Western societies. By gaining colonies like the Western powers, Japan thought that its standing and recognition in the world would rise. When they defeated the Russian Empire, the first time Japan had won a war against a Western power, it served as a marker that Japan had finally become an equal to the Western empires. This fueled their colonization, and Japan occupied various parts of East Asia, Southeast Asia, and Oceania until Japan was defeated in World War II.
As seen in the example, you can gain a much deeper understanding of history by learning about its context. This is especially helpful when you are studying a country or people you are not familiar with!
Resources that may be helpful to a history student would be
- Primary Sources. Artifacts, photos, and excerpts of the time would be helpful in understand an important component of the time. For example, a 1943 steel penny represents the wartime need for copper for military equipment and therefore used steel to make pennies instead.
- Secondary Sources. Textbooks and resources would help you get an overall understanding of the given time period.