Different 'number' in verb of relative clause using который

Question

This is in regards to Nicholas J. Brown's New Penguin book, page 336.

There, the author offers 3 alternatives, mainly the first 2 to show the difference between который and long-form past passive participles (PPP). But the 3rd sentence confuses me in relation to the 2nd because the short-form PPP in the 2nd is plural, and the associated past-tense verb in the 3rd is singular (masculine)

(in the 3rd sentence, the subject of the relative clause [from который] is the author, автор, which is obviously masculine)

1. Во всех книгах, написанных этим автором до 39-го года, есть интересные женщины.

   1a. In all the books written by this author before 1939 there are interesting women.

2. Во всех книгах, которые были написаны этим автором до 39-го года, есть интересные женщины.

   2a. In all the books, which were written by this author before 1939, are interesting women.

3. Во всех книгах, которые написал этот автор до 39-го года, есть интересные женщины.

   3a. In all the books, which this author wrote before 1939, are interesting women.

I know, for example: Interrogative pronouns taking different case than it's noun that который agrees in number and gender with the noun it modifies (in the main clause), and agrees in case with the function it has in the subordinate clause. So in both the 2nd and 3rd sentences, который is in the nominative, since it answers the question "what?" (right?).

But my question is in regards to the PPP and past-tense verb. If my English translations are correct;

2b. The PPP is plural because the object of what was written are the books.

3b. The past-tense verb in the subordinate clause is singular because the author is masculine and singular - why?. If I constructed a sentence without the subordinate clause, about a book I wrote, I would say,

3b1. Я написал эту книгу. -- I wrote the book. (assuming I am male)

3b2. Я написал эти книги. -- I wrote the books. (assuming I am male)

3b3. They wrote the book. -- Они написали эту книгу.

3b4. They wrote the books. -- Они написали эти книги.

One last question... the past-tense verb in the 3rd sentence is transitive and so should have an object, despite it being in a subordinate clause(? - Does the subordinate clause have to have an object?). The only object I see to the verb "to write" is the plural books...

Answer 1

КоторЫЕ is in Nominative in the sentence #2 (subject книги) but in Accusative in the sentence #3 (the subject is этот автор while книги is the object), but in both cases it's которЫЕ since this inflexion fits both grammatical cases Nominative and Accusative which are used for subject and object in plural respectively, it refers to книги, because they're the subject of the main clause.

...my main question was about написать... why plural in #2 and masc., sing. in #3.

Since predicate (the verb) always agrees with the subject, in #2 it's in plural (были написаны) because the subject is книги, whereas in #3 it's in singular masculine (написал) because the subject is этот автор.