Donating books: condition, and what to do with the rest
Short answer: no mould, no heavy annotation, pages and covers intact. Hardbacks in good condition typically sell better than paperbacks with cracked spines.
What sells
Clean pages, intact cover, no strong damp or musty smell (a sign of mould, which spreads to other stock).
Minimal underlining or notes — light pencil is usually fine, heavy highlighter or ink annotation isn't.
Complete sets or series are worth more to a shop than single volumes pulled from a set.
Too damaged to donate?
Paperback covers can usually go in household paper recycling; hardback covers typically need general waste unless your council specifically accepts them.
For bulk clearing (like a house move or estate clearance), We Buy Books or Ziffit will buy books in bulk with free postage — worth doing alongside a charity donation for the books that don't fit what your local shop needs right now.