Greetings! Historical theory can be a daunting area of the past to get involved with, and the reading is usually catered towards people who wish to invest some serious time and effort into researching and writing about the past. Seeing as you're a grad student for the subject, that condition applies to you perfectly! More books with excellent overviews can be found on the AH reading list for historiography, but here's a couple that I've read and are certainly worth picking up.
- History: A Very Short Introduction by John Arnold: This entry in the Very Short Introduction series is a great entry point to the "landscape" of history if you will. Whilst Arnold does not actually put forward another school of historical thought or spend too much time analysing one particular theory, he does do an excellent job of surveying the "scenery" of historiography if you will. A fairly short read (clocking in at just shy of 140 pages with sources), it will serve as a good bit of foundation and get your mind asking the questions about how historians go about studying the past.
- What Is History? by Edward Hallett Carr: The late E.H Carr's marvel of a work on historical theory, this book is definitely more on the argumentative side of writing about historical theory. Carr's delivery of his beliefs and analysis of previous historical movements is exemplary, though be prepared to spend a bit of time getting through the (nonetheless beautiful and downright magnificent) prose to the "real message" in each chapter. An older book for sure (published 1961), but one which remains influential in the world of historical theory today.
- The New Nature of History by Arthur Marwick: This book balances a discussion on the historical theories which have emerged throughout the years alongside a more general discourse on the practice of history as a discipline. I found it a bit refreshing after reading Carr's rhetoric-heavy work, though it is a sizable book (some 400 pages or so) so consider setting aside some time for just the key chapters you might find useful.
- Deconstructing History by Alun Munslow: Another sizable work here, but one which is also more for the advanced student of historical theory. Munslow's focus is on the analysis and evaluation of several big historical schools of thought (deconstructionism, post-structuralist, reconstructionism and the like). The language he uses can also be a bit confusing at times, and is definitely argument-heavy, so perhaps set aside this book for a skim and scan once you've got the foundations on historical theory down.
- The Pursuit of History by John Tosh and Sean Lang: This is again one of the more "practice of history" books, with chapters dedicated to discussions on sources, interpretations of the past, and accompanied by the relevant historiographical schools where appropriate. An interesting book nonetheless, though not as heavy on the historical theory as Munslow, Carr, or even Marwick.
- The Landscape of History: How Historians Map the Past by John Lewis Gaddis: Gaddis is focussed on what it means to "think like a historian" here and to see the world of today through the eyes of yesterday. He also does take some time (and pages) to discuss the social theory of historiography, while also discussing the other schools as well, charting their development and basic premises without getting too complicated in the rhetoric
If I had to recommend a trio of books to read, it'd be Gaddis' work, Arnold's introduction, and Carr's writing. The rest are still helpful in their own ways regardless, so the choice of where you start on this journey of historical writing is ultimately dependent on your knowledge so far and where you'd like to broaden your horizons first. To be a bit more helpful, I have the entirety of Munslow's, Gaddis', Tosh's, Marwick's, and the first chapter of Carr's work as PDF files (owned by myself) on this freely accessibly Google Drive folder. Feel free to download them and peruse at your leisure, and happy reading!