When multiple sources of morale are present (music, books, food, etc) the "total morale gain" is not the sum of all morale modifiers, which was the case in 0.D. If this is now intended behavior it is still problematic due to the way the morale modifiers are represented, which very much implies they should be summed.
Steps to reproduce the behavior:
Either the same behavior as in 0.D, or clearer communication to the player in regards to how the total morale gain value is calculated.

This is intentional, see #29396.
I do agree that there is room for improving the way the total bonus is communicated though. Any specific suggestions are welcome.
Alas. I feel this is not a change for the better.
Moving on though, I think morale representation is a bit of a mess generally, not only because the final value is now not what one would expect, but also because in the item descriptions of books the morale gain displayed is actually multiplied by 5, while for food items the value displayed is the value used.
Perhaps at the heart of the problem is the fact that morale is given as an explicit number in the first place, which is rare in a game that typically hides most of the underlying mechanics and represents them with vague indicators, if at all.
The morale menu can definitely use some cleaning up to make what's happening clearer. As for books, I suspect the inconsistency might be related to the blocks of time in which reading is done.
I'd be all for making all of the morale feedback fuzzier, ideally textual.
Most helpful comment
This is intentional, see #29396.
I do agree that there is room for improving the way the total bonus is communicated though. Any specific suggestions are welcome.