On book spines, in the UK it's one way (begins at bottom), in the US the opposite.

Actually both UK and North American spine titling runs top to bottom; it's other European countries that run bottom to top.

But to answer W's question, I would say he/she has it right if signage is going on the right side of window. If on the left side then top to bottom would seem more appropriate. In each case the baseline is reinforced by the window's edge (esp. if there are no descenders)

On the other hand, where do you expect people to be standing? If they're standing in front of the window, they will be reading the text upside down if they tilt their heads, and will need to move to the right in order to read it the right way. So having the text run bottom to top at the right edge of the window seems to me to orient the text outside the window frame, whereas running it top to bottom orients it to a reader standing in front of the window.

Very good point John! My thoughts were for foot traffic coming at the glass from outside the frame but obviously that only works in one direction! Perhaps there is a strong case here for keeping the text horizontal, placed in any one of the four corners?

From the brief description here I think I would be tempted to go for reading downwards, although I'd make my decision based upon the exact circumstances and conditions in situ. I have often wondered if there is a handedness issue with this. Looking at bookshelves for example, with both 'up' and 'down' spines, I find it irritating to have to keep rocking from side to side and find tilting to my right much more natural; I'm right handed.

Typically when you're doing vertical text you want people to read up; the reason it reads downward on book spines is so that it will be upright when the book is lying cover-side up on a table.

I agree, though, that orienting it so the baseline is supported by the edge of the window makes sense.