noun, noun which may take the genitive case particle 'no'
half-read; left unfinished
Describes a book, magazine, or other reading material that has been started but not finished; often used with の to modify a noun.
机の上に読みさしの本が置いてある。
There is a half-read book on the desk.
読みさしの漫画をそのままにして出かけた。
I left the half-read manga as it was and went out.
食べさし refers to food left half-eaten, while 読みさし is specifically for reading material left unfinished.
やりかけ is a more general term for something left unfinished or in progress, applicable to tasks, work, etc., not limited to reading.
Compound of 読み (yomi, 'reading', the continuative form of 読む) and さし (sashi, from 止す sasu, 'to stop/leave something in an incomplete state').