Final answer:
The correct adjective ending to fill in the blank is '-es'.
Step-by-step explanation:
The correct adjective ending to fill in the blank would be es.
The adjective gut is in the nominative case and modifies the noun Buch (book) which is neuter gender.
When a neuter noun is singular and in the nominative case, the correct ending for the adjective is -es. Therefore, the correct answer is 'Er liest ein gutes Buch.'
Learn more about Adjective endings in German