Please cancel this question before I make myself look more stupid. It's just dawned on me that the soak box mnst be a box of special liquid at the Post Office, where they unseal letters - ones without proper addresses on, for example.

What books would you recommend to learn more about the modal verbs? I've written here, on alt.english.usage and on uk.culture.language.english; ... I would need some reliable reference based on grammar (correct usage), and actual current usage