How to join two tables, concat_group, then group by count of that concat_group value?

I have data that looks like this:

serialized_book:

id | author | title
---+--------+---------------------------
0  | foo    | i am foo.
1  | bar    | the derivabloviating . . .
...

chapter_pub_date:

id | chapter | year | month | day | book_id
---+---------+------+-------+-----+---------
0  | 1       | 2024 | 12    | 10  | 0
1  | 2       | 2025 | 1     | 5   | 0
...
27 | 1       | 2024 | 12    | 10  | 1
...

I want a query that yields unique authors-who-published-that-month (I know the data isn't fully normalized since I don't have a separate table for "author").

So something like this:

month   | unique_author_count
--------+--------------------
2024-12 | 2
2025-1  | 1
...

The best I've got so far is

select 
    w.author, concat(p.year, "-", p.month) as yr_mo 
from 
    serialized_book w
join 
    chapter_pub_date p on w.id = p.book_id
group by 
    w.author, yr_mo
order by 
    yr_mo desc

This returns a list of every unique (author, year-month-they-published-something):

author | yr_mo
-------+----------
foo    | 2024-12
foo    | 2025-1
bar    | 2024-12
...

But I'm stuck going from this to

yr_mo   | count_unique_author_per_yr_mo
--------+-------------------------------
2024-12 | 2
...

I think it's either going to require some kind of partition or a sub-query, but when I look at examples in the docs and on SO, I can't find any that feature joins, which complicates the sub-query solution.

This is actually simpler than you are making it out to be. You can use the DISTINCT qualifier inside the author count and you won't need a subquery at all.

Select year, month, Count(Distinct author)
From serialized_book w Inner Join chapter_pub_date p
    On w.id = p.book_id
Group By year, month
Order by year Desc, month Desc