Re: Manual?
... detailed UI style guide that app developers were more or less required[2] to adhere to.
...
[2] Hedging because I forget the details of how this was (is?) enforced
IIRC it was volume 3 of "Inside Macintosh",with volumes 1 & 2 being how to actually write code and use the built in APIs.
It wasn't enforced, and couldn't be. Naturally, certain established developers - including Microsoft IIRC - tried to just port from other platforms and do the bare minimum to make it work. That resulted in programs that were modal* and had very non-standard (from the Mac UI PoV) interfaces. Except for very niche programs, the result was "user dissatisfaction" that essentially forced developers to sort things out or see their program die because a different program that people liked using ate their sales.
And that's one (of many) things that really pees me off as I have to use Windows for $dayjob. The way different programs from the same manufacturer (yes, Microshit again) have subtle or major differences in UI. it can be little things like most programs scrolling whatever window is under the mouse pointer - but Excel only scrolls if it's the active window, and it always scrolls if it's the active window. Add in that Excel hides the cell highlighting when not the active window (because you had to switch windows to scroll a different one), and it gets even more annoying. And then you get things like Outlook using totally different keyboard shortcuts to every other MS program - meaning that (since I use a Mac at home, and so have to be coping with two different UIs anyway) use keyboard shortcuts far less than I could simply because it means spending so much time thinking about what the hell I'm currently working on and so what the right shortcut is.
* In 1984 when the Mac came out, most software was modal - i.e. you worked in one mode for one thing, had to exit and enter a different mode to do something else. You could only do what was in the limited menu available in the mode you were working in. Take something simple like editing text. A typical word processor of the day had an edit mode - you opened a document and could edit it, but couldn't print it. To print, you had to save it, exit back to the main menu, and enter a print mode from where you could print a document (which you might have to select and open as it might not be remembered that you'd just been editing it.) The you'd exit the print mode and re-open the document in edit mode, and ...
The Mac introduced modeless operations - you could be typing away in your document and at any time just select Print from the File menu (or hit Cmd-P). For most developers at the time, the idea that the software had to handle user commands whenever the user wanted to use them, rather than when you permitted them, was a major change in thinking - leading to the Mac having a reputation for being hard to program.