I wouldn't worry about it yet. Whether it is a major component in your study will depend on your program and approach. But if you want something to read over the summer that will put you in a good position to think critically/theoretically about history, here is a brief and incomplete list of "classics" (the following chosen from a historiography course I took in grad school, just picking the major books — most of it was articles):

• Carr, What is History?

• White, Metahistory

• Stern, Varieties of History (a reader of lots of historical writings)

• Novick, That Noble Dream (US focused, but still useful)

• Roberts, The Narrative and History Reader (another reader)

• Nietzsche, "Advantages and Disadvantages of History for Life" (useful mostly for the Foucault)

• Foucault, "Nietzsche, Genealogy, History"

This is by no means complete but would put you on a much stronger historiography footing than most incoming graduate students to history programs. And if you really want to go full Foucault, Discipline and Punish and The Order of Things are more than enough to start with.

(I'd skip the Derrida entirely.)

I'd get a copy of 50 Key Thinkers on History. It's really a nice volume, you can kind of lightly read whatever you're interested in in bite-sized chunks, and then it'll be a great reference volume if you're assigned Foucault or Thompson or whomever.

Beyond that, I'd recommend seeking out good history books. The Books and Resources list is one place to start. You might also skim through past award winners from the American Historical Association until something catches your eye. I'm also a fan of many Bancroft Award winners, which are mostly focused on American history, broadly construed. You might also look into what professors in your field have recently published. (Just start googling "japanese historian at x university" until you get a hit.)

Basically, I'd urge you to think about investigating three angles through your own leisurely reading. First, what are the big questions that are currently driving research in your field (imperial Japan) and who are the big names. Second, what are the different kinds of things people are studying and how are they pursuing these subjects. And third, what subfields and approaches actually catch your interest.

You absolutely do not need to answer these questions before you start grad school, but if you've got even a loose sense of your interests and where those fall in the bigger picture, you'll probably have a better sense of direction as you dig into graduate work. I think my own catch that got me thinking about what I wanted to do was Before France and Germany. If I had to recommend a few books that most people would enjoy while also giving them a sense of the diversity of historical approaches, I might suggest The Unredeemed Captive, The Mortal Sea, and The Return of Martin Guerre. Those are pretty random and off the cuff, but perhaps one will catch your interest.

And I must say, pace /u/restricteddata, that I think Derrida has a lot of potential value for historians. Plus, forcing your way through Derrida helps build a mental agility that can help you grapple with most things. But I'm in complete agreement that he's not a good place to start. Unless you already have a sense of who he's talking to and what he's talking about, you're better off spending your time with Finnegans Wake.

Philosophy remains at the core of every human science. It lays the groundwork upon which you can build any theory and knowledge. It starts with three never ending question: what do you know? how do you know it? do you still doubt about it all? The more you doubt, the better. The more you question, the better.

Presocratic philosophy is a lot of fun, but it isn't considered seriously anymore. It all starts with the "holy trinity": Socrates, Plato and Aristotle. You definitely need to read about those three before entering anything else. I wouldn't advise to read them though. They can be boring at times and their language is often outdated, which makes their ideas more difficult to understand. Moreover, Socrates hasn't written anything. However, you should find an entry level book about their philosophy and their ever so long-lasting legacy.

Once you've dealt with Socrates, Plato and Aristotle, you can dive into philosophers more concerned with the matter of history. However, learning about Descartes, Pascal and Spinoza would be a must too. They deal a lot with the matter of knowledge: how do we know things? When can we stop doubting about what we know? Etc. Again, you don't need to read their work (unless you get a kick from it) but you should learn about their theories and philosophical questions.

On the matter of history itself, you really can't bypass Husserl or Heidegger. Capitalizing on the idea of progress, they theorized the "march of history" as the slow "triomph of Reason". They lived through the early 20th century so you'd think they'd be more upset with WW1 and WW2. However, they saw those worldwide disasters as a passing deflection, a way for mankind to test itself. Most philosophers, today, work within the paradigm frame Husserl conceived in his works. Ignoring him woud be a no-no.

Gellner, who's less ominous about the triumph of Reason, wrote a few books about the philosophy of history which temper Husserl and Heidegger's work. I quite like it in the way that he only fathoms two revolutions: the neolithic and the industrial revolutions. It helps to tone down the importance of events that are often way too highlighted such as the American or the French revolutions.

Many historians, though they don't publicize or acknowledge it, are disciples of Husserl and Heidegger.

They weren't philosophers strictly speaking, but medieval authors such as Augustine, Isidorus and Aquinas shouldn't be ignored. They conceived the passing of time and the march of history in a way that is at the total opposite of Husserl and Heidegger. The further we "stray" from the origin of time and its golden age, the closer we find ourself to moral decay and social anarchy. The dream of a lost golden age is still often found in less intellectual and academic historians today. "Remember, the good old time?" Therefore, it's worthy to understand the Chritian roots of this paradigm and why we don't abide to it anymore. It has found its limits, but its poetry remains quite seductive.

Less "popular" by today's standards but no less important is Marx who really placed history at the center of his philosophy. He theorized the concept of social revolution in explaining economic mechanisms in a very convincing manner. Many historians, who study "underdogs" and revolutionaries (from Spartacus to Che Gevara), make a good use of Marx's philosophical paradigm. Funny enough, you'll always find one hardcore communist in every history faculty--either a teacher or a student. Therefore, you can add Das Kapital to your to-read list.

However, a more accepted philosopher to explain social phenomenons today would be, unless I'm mistaken, Weber. Not only did he tie the "rise" of capitalism to the success of protestantism--which is being debated--but he also theorized the concept of authority in a way that is very useful to historians. Why do we obey someone? Why do we grant authority to someone? What makes a legitimate use of power? Every society would answer those questions differently and Weber laid an essential groundwork on the matter by encompassing as many types of historical human societies as he could.

If you read anything related to sociology, however, you should read an introduction on Durkheim. He basically fathered sociology from history and though his practical observations seem a bit outdated today, his philosophical concern of society as a system is still at the core of many studies today.

Historians, however, not only deal with the passing of time, its meaning, or social structures and their constraints, they also read documents. It means that we deal with language every day. Therefore we should have at best a basic understanding of it. Saussure, Derrida and Fish, for example, wrote about the language as a system and the act of understanding it and participating in it. They helped renew textual criticism and the historical method. They allow to have a more elevated understanding of language and therefore a more nuanced knowledge of facts reported in writing. We should always keep in mind that historical records are the product of their time and that at any given time, language was its own system and it was quite different from now.

I think that if you start reading about all the authors I suggested, you'll find out about many more and will be able to deepen your philosophical knowledge beyond what you need to write a master thesis or a PhD ;-) Good reading!