It would be a pleasant addition to be able to adjust which section of the block bookshelves occupy, so that they can either be sunken into a wall or placed flush with surrounding blocks, similarly to Bibliocraft's bookcases.

I actually didn't even know about the middle option until I was taking the screenshots for this suggestion, but I included it anyway for completeness' sake.

I like this idea, but because of how I have the metadata set up it would not be the cleanest to implement without breaking old worlds. I think I will definitely add it once I port to 1.13/1.14, not sure if I will add it for 1.12

I have on my notes for 1.14 to added bookshelves in the front of the block. I do not plan to add middle bookshelves at this time, but I would like to have "double bookshelves" where the shelf is in the front and back of the block