Why 볼 instead of 읽?
Koreans use the word 보다, which literally means "to see", also when they talk about reading. In fact, I'm sure using 보다 is actually more common that using 읽다.
A book to read tomorrow.
this sentence is wrong?
Nope, that's correct. You can also say "a book (that) I/you/she/he/it/we/they will read tomorrow"
And just like a real programming language, some esoteric feature that nobody knew existed can make for strange, yet completely compilable code.
Or "A book that will be read tomorrow"
볼 is so different from 읽
This should not have a period at the end because this is not a sentence.
I put, "내일 읽을 책," and it was correct. :)