Should I split these fields or leave as one?
Splitting given names from surnames is not normalisation.  What would be normalisation would be if you created a series of tables that contained additional details about some of the columns in your table.  

For example, you could create a HOUSE table that contains information about the house other than the name, such as it's catch phrase or home town etc.  You might do something similar with TITLE and BOOK.

Don't worry about nulls in your case.  Some people are concerned about too many nulls because it can be a sign that your design is not correct or because they are worried about wasting disk space.  In your case neither of these concerns is germane.