1. Will it be possible to make books in a future build? If that happens then will we be able to place those books on our bookshelves in a precise way? For some people it is fun to arrange a book shelf to their own liking.
Sounds interesting, we will keep this in mind and think about if and how we can perhaps implement that 
2. Will it ever be possible to create keys for specific doors in the game? This would be very fun to use, especially in multiplayer games. Perhaps different key styles for aesthetic purposes. Also, it might be possible to make some keys that are more difficult to pick than others. I have not quite thought that last bit through all the way, but there might be something to it.
This is planned. Especially LUA scripts (requires still suitable events for that) will be very powerful in that manner. But keys and Locks are also planned.
Why different key styles? Maybe should make it possible to break into doors with some high skill levels
Also a good suggestion, always loved lockpicking in games 
3. The posters that can be placed in game are great. Would it be possible in the future to build wood or plastic frames to put around them? Depending on the look a builder was going for this could add a lot to a structure.
Can't say for sure. It was initially planned, but then we had no more time to think about it any more^^ But @Quezaxs tip is a good workaround.
Also, you should be able to upload bigger sizes pictures (maybe not on multiplayer though due to server limitaitons)
We will add this setting to the game menu!
In multiplayer, the image size can already be specified in the server.properties file 
For a picture frame, you can use the window frame put glass in it and place the poster on the glass.
It would be nice if posters pointed at an empty frame snapped to fill it, like glass does.
Good point, we will keep that in mind! 