In the first episode of the spinoff series RWBY Chibi, Ruby find's Blake's copy of the book Ninjas of Love and starts reading it. When Ruby finds the centerfold, she says, "Now that's a katana." I understand this is supposed to be a double entendre, but it is a book about ninjas, so I'm not sure if the picture is lewd or if it really is a katana (or both). It would make sense to me that Ruby would be more interested in weapons. I'd also like to know more about the contents of the book, since it does appear in the the main series as well. I've seen comments in forums suggesting it is a reference to the Make-Out Paradise series in Naruto, but I don't know if this has been confirmed by the developers.
According to this transcript of director commentary from volume 1, Ninjas of Love is a reference to the Naruto series.
Blake holds Ninjas of Love.
Miles: Oh, I love that moment!
Monty: It's funny 'cause neither of you watched Naruto.
Miles and Kerry: No, just never watched Naruto.
Monty: There's a joke similar.
Kerry: Oh, really?
Kerry: No, I don't.
Monty: Maybe that's why you guys were wondering. Yeah, no but it's kind of a call out, too.
Based on these comments, Ninjas of Love is likely a reference to the adult-themed books written by Naruto's master. The book being about ninjas is a clue to it being a reference to the Naruto series.
According to the wiki page for books from the series, some information has been gleamed from the text on the book in the RWBY Chibi series despite the text being difficult to read: The author is Patty Berdioler, and the tagline on the front is
No shadows can conceal the lust of one enigma for long...
and the text on the back is
When a young woman abandons the comforts ... where she was bound to have an easy life but ... and self absorbed to see past her petty ... realize how lucky she was, she runs away to ... where she becomes ninja.
Little does she know that over the years she has been watched from the shadows by a ninja master, a lone enigma that haunts the darkness of the night itself. What will happen when our young woman is consumed by the heat of this shadowy man...?
Additionally, in the first episode of RWBY Chibi, Ruby calls the book "filth".
The book is probably an adult romance novel.
As to the original question, there are some reasons to assume the centerfold really is a picture of a katana or contains a katana. Firstly, RWBY Chibi is a comedic show and not canonical to the main series, so while it may be unusual for a katana to be on a centerfold, it would be reasonable for a joke. Secondly, Ruby has been show to be obsessive with weapons in the main series, particularly in episode 2. Third, it is a book about ninjas, so a katana wouldn't be out of place. In fact, a katana is seen on the cover in RWBY Chibi.
The main argument against it being a katana would be that it is a lewd book, which is compelling evidence by itself.