It's never too late, unless the deadline is gone! You have just under a month to get yourself together.
I would recommend finding some MOOCs (Massive open online courses) to do or picking up some books, pronto! Think about what you've already studied, or museums you've been to. Look for local museums.
Honestly, name dropping books isn't even a good strategy. You'd be much better served talking about a particular area of history you like, that you would study during your time at
Oxford or
The Other Place. When you talk about books in your personal statement (which I didn't for maths) you want to really talk about what you gained and how that deepened your interest. For instance, when I talked about cryptarythmns in my personal statement, I said something to the effect of:
'I recently took an online course in cracking cryptarithms - which I learned are really just an exploration of number theory. This was enriching because, while teaching me how to solve them, I also learned about how numbers and place value works. This made me think about how we define numbers, and the rules we use in the decimal system, really enthralling stuff.'
As you can see, I introduced what I did, then what skills and information I learned while doing it. I was directly showing how the activity was enriching to my maths education overall.
You really only need 2 or 3 different subject parts to talk about in your Personal Statement. If you want any help whatsoever with your application, just chuck me a message.