I excluded "sono" and it wasn't accepted. Should it be accepted?
Maybe. It can also mean 'a book' if the sono excluded, that may be why it wasn't accepted.
The English article "the" indicates a specific book, so omitting その or この would be incorrect.
Also, if you exclude the 'sono' it could means "Books are boring"
Tsumaranai vs tsumaranaka'ta??
つまらなかった is past tense.
Sono hon wa sumaranai desu.
Shouldn't あの本はつまらないです be also accepted? "Sono" and "ano" means the same.
If they wanted sono it should say that book shouldnt it?????