Postgres select all data via relation - postgresql

Question

If parents have children, and children have books they've read, how do I know all the books read by all the children of a parent?
Basic setup:
CREATE TABLE parent(
id SERIAL PRIMARY KEY,
name VARCHAR(50) NOT NULL
);
CREATE TABLE child(
id SERIAL PRIMARY KEY,
parent INTEGER REFERENCES parent(id) NOT NULL,
name VARCHAR(50) NOT NULL
);
CREATE TABLE book(
id SERIAL PRIMARY KEY,
readBy INTEGER REFERENCES child(id) NOT NULL,
name VARCHAR(50) NOT NULL
);
Insert some data:
INSERT INTO parent (name) VALUES
('Bob'),
('Mary');
INSERT INTO child (parent, name) VALUES
(1, 'Stu'), -- Bob has children Stu and Jane
(1, 'Jane'),
(2, 'Greg'), -- Mary has children Greg and Bella
(2, 'Bella');
INSERT INTO book (readBy, name) VALUES
(1, 'Crime & Punishment'), -- Stu has read C&P and Goodnight Moon
(1, 'Goodnight Moon'),
(2, 'The Singularity'), -- Jane has read The Singularity and 1fish2fish
(2, 'One Fish Two Fish'),
(3, 'Narnia'); -- Greg has read Narnia (Bella read nothing)
How do I formulate a SELECT query involving "Bob" as a parameter and get all the books read by his kids?:
( 'Crime & Punishment', 'Goodnight Moon', 'The Singularity', 'One Fish Two Fish' )
The same query, except involving "Mary" should give only the one book read by "Greg", who is her only child who has read anything:
( 'Narnia' )
Thanks in advance for any help! :)
Disclaimer: I'm sure this question must have been asked before but I wasn't able to find it :(

Answer 1

You can chain joins
select book.name
from book
join child on book.readby=child.id
join parent on child.parent=parent.id
where parent.name='Bob';
Or if you want the results as an array/list
This is unusual, usually results as rows like above are more useful but you appear to be asking for a single-line result.
select array_agg(book.name)
from book
join child on book.readby=child.id
join parent on child.parent=parent.id
where parent.name='Bob';
Note: the results could appear in any order.