y-* tags categorize dreams.
For types: beyond the obvious, there are dreamlets (very short dreams), stubs (fragment/outline of a partially-lost dream), gnatter (residual impression of a lost dream).
For characters: there are roles (characters fitting an archetype), symbols (characters as symbols), and sigils (recurring figures with a significance bigger than a single dream's role/symbolism).
x-* tags categorize books.
Material is categorized primarily by structure, style and setting. If searching for a particular genre, look for the defining features of that genre, e.g. x-form:nonfic:bio, x-style:horror, x-setting:dystopian.