sebuah is a classifier which means a.... it is really formal Indonesian and is not usually used . The classifier changes depending on the a variety of thing sebuah, seorang, seekor,

https://forum.duolingo.com/comment/3842014

Sebuah indicates that you read a book rather than more than one book ... the emphasis is on what you did. If you said, Saya membaca satu buku, the emphasis would be that you read ONLY one book. For example, 'What did you do on the holidays?' Saya membaca sebuah buku. Without sebuah buku may be plural.