Is there a right or wrong way to decorate a bookcase with books/items?

I think that answer is no. No wrong way. You should do it tbe way you like it. Personally I do mine all mixed up. Meaning I have some books upright and some flat. I mix in small plants and knick knacks etc for character. I also mix up the books in sizes and cilir to make it more interesting.

There is no right or wrong way to decorate a bookcase! Most designers will tell you to keep in mind the "threes" rule - using like items in multiples of 3 so you could put 6 books standing with 3 glass objects on the shelf. But the bottom line - do what looks good to you!

There is no wrong or right...just use yourimagination and group books, frames, figureines, vases, etc,in interesting and eye-pleasing ways.

Don't place book after book after book. If you have a set of books or books

of the same genre, place them together. At the end of the series place 3 or 4

flat on the shelf with a small sweet object on top of the flat ones Leave some

space and repeat across the shelf sometimes using book ends that are

attractive or if plain, put a vase or other interesting object at the end of

the space. On descending shelves follow the same idea making certain not

to duplicate the shelf above. You can add framed pictures, sea shells.

A precious rock or object a child or grandchild gifted to you.

Definitely no rules! But my personal preference is to break up the books with some decorative items just for it to feel more homey.