ANYFREEBOOK searches Project Gutenberg, Open Library, the Internet Archive, Google Books, PubMed Central, DOAJ, LibriVox, OpenAlex, Wikisource, Europeana, Zenodo and the Open Textbook Library at the same time. (The public API fans out to all twelve. The website's own search page leaves LibriVox out, because an audiobook is usually not what someone typing a book title is after — that source has its own page.) The parallelism is the easy part. The part that took real thought was deciding what to do when one of the twelve does not answer — because the default behaviour of most multi-source search code is to quietly return a shorter list, and a shorter list is indistinguishable from "that is everything there is".

There is no search index

Start with the constraint, because it dictates everything else. ANYFREEBOOK does not maintain a search index. We keep a curated catalogue of 258 books across 241 categories in src/lib/data.ts , and that catalogue powers the category pages and the per-title detail pages.…