I found out that Castle actually does have books. Why did NCIS not do this?

Well for one thing there was already a book out called Deep Six before N.C.I.S. introduced the Deep Six books so CBS can't really write the Deep Six Series because of that otherwise they would.

their hadn't thought of it yet but their should.

There is a series of NCIS books by Mel Odom. I haven't read them yet, and I know they are not the same characters. I'll let you know how the books are once I get a chance to read one.