I picked up a book from the library recently called The Northern Crusades by Eric Christiansen, and it seems like a pretty interesting read so far, but I was a little surprised to realize I was reading a book written in 1980, when they still called Saint Petersburg Leningrad and where fascism was still in living memory. I realize that old books can't be updated when new discoveries occur, but are there any special steps historians take when dealing with these sorts of documents?
Every author has their biases, and is limited by the information available at the time. Generally speaking, old books often have information - or viewpoints! - which are more out-of-date than newer books which can represent the cumulative scholarship of the intervening years. Howevever, this is not always the case, and just because a book is old doesn't mean it can't still be a valuable resource - it just means that, as the old joke goes, you have to know where the rocks are.
To take an example from my field, the first full biography of H. P. Lovecraft was Lovecraft: A Biography (1975) by L. Sprague de Camp. It was the premier biography on Lovecraft for about twenty years, until S. T. Joshi's H. P. Lovecraft: A Life (1996) came out - and there are still people that cite it today, even though Lovecraft biographies have proliferated and, towering above them all is Joshi's own 1,000-page two-volume I Am Providence: The Life and Times of H. P. Lovecraft (2010).
From it's publication, de Camp's biography has received both praise and criticism. It was the first full-length biography, and made extensive use of Lovecraft's correspondence and papers, as well as conversations or correspondence with those of Lovecraft's friends and associates who were still living at the time. Not all of this material is used (or cited) well, and de Camp was strongly criticized for his approach to Lovecraft, which was often harshly critical of the subject and sought to both psychoanalyze and condemn him - but it also strongly approached the issues of Lovecraft's sexuality and racism, not for the first time, but in more depth than anything that had come before.
Is it outdated? Yeah, pretty much. More material has emerged on Lovecraft's life and letters than de Camp knew. Unless you're a specialist that knows what he got wrong and where, it's easy to take him at face value - and that can (and has) led to a few people repeating errors. More critically, there's the issue of de Camp's critical and psychoanalyzing approach that is problematic. Which is why I'm always leery when I crack open a new article on Lovecraft and see they're referencing de Camp. It's not that you can't do that...but folks that often do it aren't citing it with an eye for what it is: a work of Lovecraft scholarship as it was forty-five years ago. It's still good to have a copy around simply because of how influential it was on the field, because you can thus look up the actual pages when someone cites it, but it isn't my go-to when I need a biography on Lovecraft.
By contrast, just because a Lovecraft biography is newer than de Camp's doesn't mean it's better. W. Scott Poole's In the Mountains of Madness: The Life and Extraordinary Afterlife of H.P. Lovecraft (2016) is a steaming pile of a biography, full of errors and critical readings that might make even de Camp blush. But again - you kind of have at least some command of the source material (mostly Lovecraft's letters, but also various memoirs of friends and family, and also previous biographies like de Camp and Joshi's) to be able to spot where Poole is making an error (and, since he doesn't really cite sources, where he's grabbing some stuff from). I'd recommend people cite de Camp's bio only with caution, but I wouldn't recommend anyone cite Poole's bio.
I don't mean to single Poole out maliciously here, I should add; there are a number of biographical treatments of Lovecraft out there of various degrees of merit, and a lot of the more recent ones (most everything post I Am Providence) is basically just a pop-biography trying to summarize the information for fans rather than serious scholastic works. I use him as an example, yet I could just as easily have mentioned Paul Roland's The Curious Case of H. P. Lovecraft (2014) or Donald Tyson's The Dream World of H. P. Lovecraft: His Life, His Demons, His Universe (2010). Yet when you are getting started in your research, these are the kind of books that people tend to reach for.
There usually aren't guides to these sort of pitfalls. It's a judgment that comes with deeper study of the primary sources, and broader study of the secondary literature. And it is a primary reason why academic reviews are so important: if you get somebody qualified to talk about a work and weigh in on it, you have a better idea of how it stacks up. Reviews of older works are less common, however. You have to really be aware of the current state of scholarship to properly evaluate how well (or badly) some of the older works remain viable to the subject.