How can I use a bookcase as an upper kitchen cabinet?

Without seeing the bookcases and knowing how they are made makes it hard to answer whether they would support dishes, etc. You could mount them by using L brackets top and bottom as long as the brackets were fastened to studs and the brackets could be fastened securely to the bookshelves with no chance of pulling out from weight but then you would see the brackets. You would not need to add doors--this would add to the weight--but you would have to be tidy without doors! Sounds like a great cost saving idea if you can make it work. You might consider looking for some used kitchen cabinets at local thrift stores for a reasonable price. You might be surprised at what you can find to fit your budget!

You wouldn't have to add doors and go with an open shelving look. My biggest concern would be to make sure it is attached securely to the studs in the wall. You could all luan plywood for a back and that would give you plenty of surface to screw into the wall studs.

Make sure the bookcases are deep enough for what you want to put on the shelves. Dinner plates could be a problem for instance.

I still don't know how to hang them and some bookcases can't be hung at all. Bookcases of course are usually less than upper cabinets. Need doors..in my house all need doors . This is going to be part of my coffee bar. The pic comes down, cabinets on both sides and shelves in the center. It's only 18 inches deep so I might have to find 10 in deep bookcases. Might come out too far. The cases will sit above the backsplash which is tongue and groove and chalkboard painted.

It could be open. I would not add a mismatched door.