I'm purely guessing here, but also considering I read him and Linus both say "we have enough kernel developers", I think it's likely they don't want to encourage low quality contributions from new developers.
Wouldn't it be helpful then to put out more information on how to be a good contributor? I'm not sure how a technical book about the kernel would lead to worse contributions, you'd think a lack of readily available information and educational material would do that.
