Ah, then you are out of luck. You can only have one default open behavior. What you are thinking of is you used to be able to make a selection in the app and in the device's settings menu that allowed you to override the default behavior so that if you clicked the "Guide" button anywhere, it would open to the guide and you could make your default behavior to open to wherever. This was also true of other remotes with additional buttons (Red, Green, Blue, Yellow) that can be programmed with different functions to open a specific area of the app.
All of these work INSIDE the Channels app, but none of them work OUTSIDE the Channels app. This was caused by a change Google made and, in order to be compliant, the Devs had to remove that feature. The whole backstory and whatnot is in the above linked thread, too. So basically, you have one default button to open the app to whichever area you are most likely to want it to open to (I'd say the Library in your case) and then once you are inside the app you can press the "Guide" button to have it go to the Guide. Yes, it is two presses instead of one, but it is what it is.
