For books, the two that pop to mind, a half-century apart are:

European Armour: Circa 1066 To Circa 1700 by Claude Blair - 1958 This book, written when the author was a fairly young man, codified a lot of modern armour scholarship. It combined reviews of pictorial depictions, interpretations of surviving artwork and period accounts to produce a pretty complete portrait of armour during the Middle Ages and early modern period. He prefers contemporary terms to victorianism, which is a tendency that has stuck in the rest of the field (even if it hasn't filtered into popular culture).

Alan Williams - The Knight and the Blast Furnace - 2003 A systematic study of the metallurgy of early modern and medieval armour, and a brief history of its manufacture and the way its development followed metallurgical advances (hence the title). Changed the field by demonstrating that high-quality late medieval and early modern armour was heat treated to considerable (if uneven) hardness.

If you're doing any work on Irish immigration to the US (and to be honest I think there's a lot of value just a strict case study of immigration in general) Emigrants and Exiles by Kerby Miller is one of the best sources you can find. I can't tell you how many times I've pulled from it in the past few years, it's just so comprehensive. Granted I'm only an undergrad, but every professor I've talked to about it definitely agrees that it's bar none one of the most useful sources on the subject out there.

Keeping in that train, "The Polish Peasant in Europe and America" by Thomas and Znaniecki is a useful analysis of Polish immigrants, but perhaps even more important it helped pave the way for content analysis techniques and shape the Chicago school. Admittedly I haven't read all of it myself, but it's value is pretty unquestionable.

Crabgrass Frontier by Kenneth Jackson is basically required reading if you're doing anything involving urban development, and is definitely suggested if your field takes you into cities in general.

Actually in that vein "The Significance of the Frontier in American History" by Frederick Jackson Turner is obviously also pretty damn big. I won't say I fully agree with the Frontier thesis, but God damn is it an important thing to know anyway. It directly set the tone of American historiography for several decades, and has had more indirect effects since. Again, doing anything involving 19th century America without having a base familiarity with it is definitely not advisable, if for no other reason than because your sources almost undoubtedly will reference it or reference someone that references it.

Plunkett of Tammany Hall by William Riordan and all of Alfred Henry Lewis' biographies of Tammany men are pretty fantastic too. Plunkett especially is sensational, but the window they open up to the machine is really valuable even if it is a bit colored. A lot of people don't cite them, but I highly doubt any expert in machine politics hasn't taken the time to read them.

My thesis is on the relationship between political party machines and immigrants in the 19th century, so I've been drawing from all over lately.

Also, shout out to Grand Old Party by Lewis Gould and Party of the People by Jules Witcover for being some of the best histories of their two respective parties. I don't know if seminal is the right word for them, but they're definitely some of the best around.

In my actual area of research (which is really only what I feel qualified to comment on) the current scholarship is a bit of a mess.

For the longbow the best book right now is probably Strickland and Hardy's The Great Warbow which is a perfectly good book, but despite it's name it really works best as a history of the English archer rather than of the longbow. While it was described as an attempt to update Hardy's (non-academic) book The Longbow it really feels more like an update of Jim Bradbury's The Medieval Archer. It's still a good book, just not the seminal work on the longbow that it could have been.

The crossbow as a technology really does not receive much scholarly attention, especially in English. The best book on the subject is probably Josef Alm's out of print book European Crossbows: A Survey which was written in the 40s and translated from Swedish in the 70s. The best book on composite crossbows is Holger Richter's Die Horgenarmbrust which, as you might have guessed, is available only in German. It's a great book but it is also only concerned with one type of crossbow while Alm covers wood, composite and steel crossbows. Alm's book is still really good but it's old, and quite short so it could use an update.

For Pius XII, the recent biography from Robert Ventresca (Soldier of Christ) is the gold standard. It is a work of history, rather than a polemic like most of the other books on the subject.

For the Spanish Civil War, Antony Beevor's The Battle for Spain deserves every bit of recognition it has garnered. For violence against noncombatants, Paul Preston's The Spanish Holocaust provides the best insight into this sanguinary portion of history and its minor flaws (pro-Republican bias, a poor title and a thesis that isn't fully supported by his argument, and conflating various Catholic thinkers into a monolithic entity in his summaries) are outweighed by how well Preston has compiled the various sources available and its attention to detail.

My field is Museum Studies. I would say that the seminal text is Interpreting Our Heritage by Freeman Tilden. Much of our modern concept of heritage interpretation is based on Tilden's work in communicating significance through direct experience. His principals of interpretation are still absolutely valid and useful, even sixty years later.

"Through interpretation, understanding; through understanding, appreciation; through appreciation, protection." - Freeman Tilden, 1957

Made me wonder, what are the controversial new texts in the field ? (I'm always worried about trusting the past too much)

In aboriginal studies much of the history we look at comes from oral tradition but there are some major articles that are very useful. The Royal commission report on aboriginal peoples is a very comprehensive report covering a lot of traditions and culture of aboriginal people of Canada. I'd say that is probably once of the most used reports for academic writing but I expect the new truth and reconciliation report will be a major player in the field.