You are probably applying one of the many games list filters (ie: installed games) - choose the game library you wish to view from this menu at the top of the steam window (the same way you would filter to show only installed games) and this will present you the available games from that library:

(On a train at the moment so higher quality screenshot will be added later)

As Scott Weldon said, there doesn't seem to be any way to do this other than disabling sharing of the friend whose library is in use, or using offline mode as Jaxir said to prevent the library from being in use in the first place. You can change the order in which your friends share libraries with you by having them unshare and reshare the library with you, in order to play your games with the library of whichever friend is least likely to be using it, but for a general solution to choose which friend's library to use, you'll need sfs-select.

SFS-select is a python tool to enable and disable friends' shared libraries by modifying the local cache remembering which accounts have authorized you. It's a bit of a clunky workaround, but it lets you pick exactly which friend's library you would like to use for a shared game, without needing your friends to log in to your computer again, or changing the actual permissions registered on the Steam server at all. It includes both graphical and command-line versions. You can download it here or read about it on the Steam forum thread.