In Jasper Fforde's lovable literary-themed fantasy series, humans dash in and out of famous books, stopping for coffee, say, at the bun shop from ''Little Dorrit.'' This third entry finds detective Thursday Next chilling out in the Well of Lost Plots, the oasis in ''BookWorld'' for characters in unfinished novels. There she uncovers a conspiracy surrounding the ultimate ''Book Operating System'' and dodges grammasites, adjectivores, a ''mispeling vyrus,'' and a hungry Minotaur. At times The Well of Lost Plots seems to tread water, but Fforde's inventiveness remains a bookworm's delight. The highlight: a page-turning murder attempt on ''Wuthering Heights''' Heathcliff.