Depending on the specific topics of the medieval books they may not be outdated, but they probably are. I would honestly raise an eyebrow to any book came out before the 1980's at the very earliest.

All that being said, older books are useful to scholars. Many classics of the field should be, at the very least, examined to understand the contours of the debate. Furthermore, older books often have useful information, even if the models are badly outdated. For the layman of medieval history, however, stick to something a lot newer than the 1960's.

Generally, it varies. In fields that are more fluid or experiencing a rebirth, a book even a decade old can be out of date but other time periods (such as one of my focus areas, the Napoleonic Era) can still use a book produced in the 60s (in this case, Chandlers Campaigns of Napoleon). Even then, the field of the French Revolution is constantly in flux while the Napoleonic Era is relatively stable.

Defining a cut off for the pre-Columbian Americas is incredibly complicated.

Much of this is because the term ¨secondary source¨is only somewhat relevant in archaeology. A lot of your older sources are either excavation reports or ¨travelogues¨of Europeans trekking the Andes or jungles of Guatemala. To call these secondary sources suggests that you could go one step more direct. That´s not really possible. Archaeology is an inherently destructive science- excavation requires much of the ¨primary source¨ to be destroyed. For many sites, the excavation reports are the best data you´ll have, no matter how old. You can´t exactly go back and ¨re-excavate.¨ Likewise, many of the travelogues record monuents before excavation or looting, or even local oral accounts that are since gone. In the past year, I have used US Dept. of the Interior reports from 1868, Charles Wiener´s Perou et Bolivie from 1880, Wendell Bennett´s reports on Tiwanaku from 1934, and work by Julio Tello on the Peruvian highlands from the 20s. The DoI reports, for instance, are the only documentation of certain Mississippian graves and mounds in the Ohio River valley. Tello´s drawings of some Chavin monuments record details since lost to vandalism. That´s not to say that the conclusions presented in these reports, the ¨real¨secondary sources, are accurate. But it does mean that reports will be cited for a hundred years.

Now, what about books and articles and such? For the Inca, which Restall primarily deals with, there´s some quality stuff that´s a few decades old. So much of our understanding of the Inca is from historical accounts which have been known essentially since their publishing in the 16/17th centuries. I don´t work too much with the Inca, but when I do, I have used stuff from the 70s. As has been brought up in another recent thread, the Inca empire´s conquests were so incredibly rapid, and it´s own subsequent defeat so soon afterward, that archaeological investigations aren´t constantly revolutionizing anything. It´s certainly different for other cultures in South America. But it´s also not a problem you´ll run into. Andean archaeology beyond the Inca has really only exploded to the scale of Mayan or Classical studies in the past 20-25 years. So if you´re studying the Moche or Tiwanaku or Nazca, there´s a very limited amount of extant material, which is usually notably cursory or filled with uncertainties.

Edit: Just reading a piece from this year that sites a Zuidema work on the Inca from ´64.

I used a book published in 1970 for my history thesis last year. I couldn't find the information in any other published book in my college library or online nobody seemed to have a problem with it. I got an A2 for the thesis which incidentally brought my GPA to a 2:1 :)

I also used a book published in 1885 as the author had access to documents that had since been destroyed in a civil war. Although I stuck to recorded facts with this source and avoided any of the author's interpretation.