There are many ways to research a location and a time: Books, the internet, even satellite photos and Google Street View. (Not really relevant for this project, but I've fixed some pretty basic errors with those.) However, when the place in question is interesting, you need to make sure that your research is actually relevant to the novel.
When you're doing the research, ask yourself: Is this material I'm reading answering a specific question or providing critical background? Or am I just reading this because it's interesting?
There's nothing wrong with reading for deep background, or letting yourself get lost in historical details. But when you have a project in mind, you have to be careful not to do that too much. If you find yourself reading about stuff that's a bit too far from the plot and background you need info on, then put the book down.
How do you know where that line is? Well, this is something you'll need to learn to judge for yourself.
What if the book you're planning to write has a very vague plot, or you don't even know the plot? Well, if that's the case, you have a bit of a chicken-and-the-egg situation. Force yourself to outline some of the story. Even if it's vague, that plan will help you limit your research. Keep in mind that you can always make changes to the plan later on.
The fact that you have two specific eras in mind, however, says to me that you have at least a partial handle on the book's specifics. Remember that it's easier to change something than invent it out of whole cloth. You can do an edit pass later to find details that you may have gotten wrong.
But always ask yourself: Is this research inspiring me to write something? If the answer is no, you're not researching, you're just reading for fun.