My area of specialisation is imperium in the Roman Middle Republic, particularly the use and abuse of it by consuls and praetors sent abroad. Turns out these guys did everything, with at the extreme end stuff like starting unprovoked wars for profit and signing a peace just to prevent the next guy from being able to profit.

I consider A.M. Eckstein's book Senate and General: Individual Decision-making and Roman Foreign Relations 264-194 BC (Berkeley 1987) to be essential in this regard.

My area of interest is late 18th century Spanish paintings by people who aren't Goya. For a comprehensive overview, Jose Luis Morales y Marin's Pintura en Espana: 1750-1808 is hard to go past, but if you're looking for a text that will introduce you to (and hopefully make you fall in love with!) the art of that period, the exhibition catalogue Painting in Spain during the later eighteenth century (edited by Michael Helston) is actually a great one.

I tend to focus a lot on Canadian nationalism, as well as Anglo-Canadian relations and Canada's role in the British Empire. The seminal text here would have to be The Sense of Power (1970) by Carl Berger. Explains that Canadian imperialism of the early 20th century is best understood as an expression of Canadian nationalism: not just about King and Empire but about growing within an Imperial context.

I feel that late antiquity as a whole is still very niche, if the comments on Reddit about the Christian Dark Ages are representative. I hope it's not true, but I imagine it will still be relatively obscure for many people. For an introduction, Peter Brown's The World of Late Antiquity (1971) is the book to read, as it is very readable and has a lot of illustrations to show how vibrant the late-antique world was. It really is a great way to overturn all the misconceptions about the 'Dark Ages', especially as it was Brown who basically create late antiquity as a field in the first place. For Byzantine history, I really like Judith Herrin's Byzantium: The Surprising Life of a Medieval Empire (2008), which is another book aimed at a popular audience and presents a very readable introduction to the complex world of Byzantine studies.

I'm also fascinated by the Arab Conquests, so Robert Hoyland's In God's Path: The Arab Conquests and the Creation of an Islamic Empire (2014) is another of my recommendations. There are so many misconceptions and outdated ideas out there about this important period of history (reading other answers here on this topic really gets to me sometimes...), so a readable and up-to-date narrative history of this topic is absolutely essential; I think Hoyland did a very admirable job with his book and it is definitely worth taking a look at. In fact a lot of the narrative you will find in this book will contradict what you will find on stuff like Wikipedia, which is just a telling indication of how different academic history is compared to 'popular' history. Even now massive revisions are happening and I personally know of people working on proving the non-existence of certain invasions and reshuffling the chronology further, so it is a very exciting period to read about. Hoyland wrote this book with a popular audience in mind though, so it's not a dry and complex book at all. If I have to recommend one book, I will definitely suggest that you read this one!

My main focus is the Carolingian period, primarily Charlemagne. A good starting point though would definitely be Ian Wood's The Merovingian Kingdoms and Rosamond McKitterick's Charlemagne: the Formation of a European Identity is also a solid work, although it does have some pretty specific stuff that I feel only medievalists would necessarily be interested in.

History of pornography and obscenity.The Secret Museum should be read by everyone who wants to know about both museums and the history of the obscene.

If you're interested in privacy and family, my other specialization, stone's Family Sex and Marriage in England is great.

I have a few areas that I work in, and I think of myself as a specialist of overlapping disciplines. Perhaps my most core methodological approach is environmental history; for that, one of the most essential texts has to be William Cronon's Nature's Metropolis: Chicago and the Great West.

I specialize in demography and disease in the protohistoric New World. My very strong recommendation for people who want to start reading about the Americas after contact is Restall's Seven Myths of the Spanish Conquest. The book examines the various myths, inaccuracies, and assumptions that permeate the popular version of the history of conquest. Simply put, the book rocked my world the first time I read it.

David M. Glantz's "When Titans Clashed" is perhaps the best introductory text for the Eastern Front of World War II. Accessible style, but not dumbed down. Can't go wrong with it.

I'm a specialist in "Shinto," meaning Japanese shrines and mythology. Nearly all information available about it is misleading (don't even Wikipedia), and it's only in the past 5 years that important studies have emerged about the Japanese conceptualization of "religion" and how it differs from the West. Recent publications like Breen and Teeuwen, A New History of Shinto (2010) and Josephson, The Invention of Religion in Japan (2012) are going to be cited for years to come.

Ancient West Mexico - Art and Archaeology of the Unknown Past (1998) edited by Richard F. Townsend. This volume of work is one of two books most commonly referenced for studies of shaft tomb figures associated with the shaft tomb tradition and Teuchitlan Tradition of West Mexico. Townsend and contributing authors provide a wonderful array of papers and essays discussing these mortuary items that have been heavily looted in the past. This volume, unlike Von Winning’s, provides the reader with wonderful and large color photographs of figures. Information about the figures have also been updated since Von Winning’s work, but any inquiry into the field will show that progress has been slow due to a lack of excavations, partly due to the rampant looting and partly due to researchers unable to find tombs to excavate.

I'm starting to specialize in Southern history, primarily 1840s-1900. I would highly recommend Reconstruction by Eric Foner. Woodward's Origins of the New South and the Simkins biography of Tillman are also very good

Philip Jones's From Commune to Signoria is exhaustive, massive, expensive :(, and the standard narrative in English and will be for some time.

Robert Chapin Lane's Venice: A Maritime History is old, but still very good. In his recent popular history of Venice (also pretty good), Madden feels the need to name check and compliment Chapin Lane.

Steven Epstein's Genoa and the Genoese is the only survey history of the Ligurian republic.

Najemy's history of Florence is the standard history of medieval Florence.

Augustine Thompson's Cities of God is just a straight up amazing monograph.

Victorian prostitution and pornography is my bag (I wrote a dissertation about it...nbd). I would read Judith Walkowitz's Prostitution and Victorian Society. That was kinda "the first" to really take an indepth, academic look at this, right when women's studies was coming into it's own.

Michel Foucault's History of Sexuality, vol 1. It's a challenge, but mindblowing.

Love in the Time of Victoria: Sexuality and Desire Among Working-Class Men and Women in 19th Century London. By Francoise Barret-Ducrocq. She looks at a Foundling hospitals archives--eye-opening and heartbreaking look into the love lives of ordinary people.

Finally, I did enjoy The Other Victorians by Steven Marcus. Academically speaking, it's dated, but he was one of the first to take pornography seriously.

I'm an historical archaeologist. Very accessible is In Small Things Forgotten by James Deetz.

Unearthing Gotham by Diane diZerega Wall and Anne-Marie Cantwell is a beautifully written book about urban archaeology in America's most famous city.