All very interesting Rustic.
Sometimes memories can all come in a rush like this. It's important to write it all down, because, they can just as suddenly dry up. Who knows why? It happens. When you're in the 'drought' that's a great time to go back over your notes and look for connections and patterns and ponder how all this relates to your present life.
Personally, I suggest not doing too much (any) googling and checking historical facts and dates and so forth at this point. I'd just focus on getting your memories down on paper with as much detail as you can for now. By all means include images of staircases, castles and so on, that look like what you saw and so forth. The book turning up at this time, with an image of the exact staircase you remember, is quite a coincidence. Strange coincidences like this can certainly be significant.
Although checking historical facts can be fun, if you do it too early in the piece you can confuse yourself, or end up jumping to conclusions or going down a blind alley where you think you know what's what when you might actually be quite wrong. It's much better to work with the snippets of information you have, meditate on them and see if you can get more detail on them on your own. (By the way, many people believe that you can experience two lives in the same historical period. Time is a funny thing and doesn't necessary 'behave itself' the way people think, but that's a whole other thread).
You'd be surprised how much information is in your 'inner google'. Although most of your information comes from dreams, you can access information in a 'half-dream' state with a bit of practice.
Just close your eyes and relax. Slow your breathing. Clear your mind for a few minutes. Stop thinking. Don't worry about what will or won't happen. Just relax. Then, when the 'internal chatter' has stopped (or at least mostly stopped), think of a bit that you do have. Just choose one scene that seems the most powerful or potent. Picture it very clearly in your mind. Go over it again in your imagination. Then 'rewind' and 'replay' it a bit slower. Ask yourself some questions such as: Who was that lady? What did she mean to you? Why were you there? What was your name? Does a date spring to mind? What happened next? Why is this moment significant enough to remember? etc. etc. Go with whatever comes even if it seems outlandish or silly at first. Trust it and see where it leads you.