"Tienes demasiados libros en la estantería."

Translation: You have too many books on the bookshelf.

[emc1995]

'You have too many books in the bookshelf' should maybe be accepted.

[Andreaja69]

'On' is the preposition that is used with 'bookshelf. The books aren't inside the shelf, they're on top of it.

[roselaw]

That's not entirely true. Many people use bookshelf and bookcase interchangeably. For instance, this is from Wikipedia: "A bookcase, or bookshelf, is a piece of furniture with horizontal shelves, often in a cabinet, used to store books or other printed materials." So something can definitely be in a bookshelf.

[GringoAmigable]

En = on or in. I have lots of books in my bookshelf.

[Joseph-Marsh]

I'm a native English speaker and I would say "in the bookshelf," not "on"

[Joseph-Marsh]

However, I would say on the shelf if the shelf is not part of a bookshelf or cabinet

[W.Ty.S.]

Native english speaker. I personally say on and think that sounds a bit more natural (regardless of bookcase or bookshelf). However, if I heard someone said in, it would totally pass without my notice. Probably best not to over think this one.