Black's Writing Dictionary is widely used in adult literacy classes and in schools. It's a good choice for those who find ordinary dictionaries challenging. This is a relatively conventional dictionary that concentrates on the needs of those who use dictionaries for spelling, rather than for meanings, giving very simple definitions of words.
Unlike the ACE (Aurally Coded English) spelling dictionary there is no 'system' to learn to look words up. This dictionary can be used instantly.
It's not as sophisticated as Chambers Adult Learners' Dictionary and has far less content but as a result would be an ideal choice for reluctant users. The paper used is quite bright, white paper which may present problems for some users with sensitivity to this.
This is not a dictionary that will improve the writing of those who are already reasonably fluent. For that Chambers would be a better choice.