I've both asked and been asked this. It's unusual in my experience (US, tech), but certainly not unheard-of. But what and who you ask matter.
The HR person is probably not going to be able to field this request. You really need to connect with the person who will be your manager. If you already have contact information for that person, you can send email such as the following:
I'm looking forward to joining your team on $date. I'd like to get a head start on the technologies1 we'll be using. What would it be most important for me to read up on? Is there anything specific that it would be helpful for me to review?
1 Or domain, if that's more important.
This question is focused enough to be answerable ("such-and-such library is really important", "you might want to get an early start on this hefty legal thing", etc), but broad enough to let the manager steer.
If you don't have a way to get in touch with the manager, then you can ask the HR person to redirect you. Something like:
(some sort of pleasantries)
Would you be able to put me in touch with the person who will be my manager? I have some free time before I start and was hoping to use it to get familiar with the technologies we'll be using, and I'd like to ask which are most important to focus on.
What you don't want to do is to ask the HR person what technologies you should learn, or what domain documents you should read, or whatever. That's not his field, and you don't want to start off with the HR person telling people, including your manager, about the new hire who doesn't even understand basic job functions. Your request to the HR person is for an introduction, or to have your request forwarded; that's a reasonable request of an HR person and shouldn't raise eyebrows.