If you're looking for academic history, the review section of your field's journal is always a good start, as is the American Historical Review. If you want something more popular, the NY Times Review of Books often has some of the more interesting (if also sometimes more provocative) up and coming titles in popular history.

If there are specific publishers who tend to publish in your field, subscribe to their updates.

A less obvious medium for keeping up with new titles is Twitter and other social media. You can follow publishers, journals, and scholarly societies, but I often see interesting new books popping up via big names in my field who have a strong social media presence.

In addition to what's already been said, I also make it a point to check publishers' websites every couple of months to see new and upcoming releases. Generally, I check the "big" academic publishers: Oxford, Cambridge, Yale, California, North Carolina, and a few others. Checking these websites is much better than even Amazon at finding new/upcoming releases, and it's also easier to find books within your specific field of interest.