This is not a great solution since it requires GUI scripting but it does work:
tell application "System Events" to set the_name to name of window 1 of process "Microsoft Excel"
tell application "Microsoft Excel"
repeat with i from 1 to count of documents
if Name of document i = the_name then return document i
end repeat
end tell
Microsoft Excel has a substantial dictionary, but doesn’t follow the standard AppleScript Object Model - it seems more Visual Basic based, which isn’t a surprise.
As a result, the information isn’t where you’d expect it to be.
The following code will return the active/frontmost window:
tell application "Microsoft Excel"
get ActiveWorkbook
end tell
As for where you’d find this command in the dictionary, check Custom Suite → Application and you’ll see ‘ActiveWorkbook’ as a property. Technically, of course, this is an application property but it’s not in the standard suite for some reason.