机の上に読み___の本が置いてある。
Q. What kind of book is a 読みかけの本?
Translation
There's a half-read book lying on the desk.
Explanation
Masu-stem + かけ means an action begun but left unfinished: 読みかけ 'partway through reading'. It contrasts with 読み終わった (finished) and まだ読んでいない (unread).