Last updated 07-30-2026
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Book Detective (formerly What is that Books?)
Book Detective is an AI book search engine that finds titles from vague plot memories, character details, or mood descriptions you type in chat. It started as an experimental tool called What is that Book on Joelbooks and now runs as a custom GPT in the OpenAI GPT Store with a curated dataset of 10,000+ titles beyond ChatGPT's training cutoff.
Keyword book finders choke on fuzzy prompts like "fantasy with dragons and a clever heroine." Book Detective combines GPT-4 with Joelbooks' editorial dataset of fresh fiction and nonfiction excerpts, updated weekly with upcoming releases ChatGPT alone cannot see. The trade-off is that it lives inside ChatGPT rather than as a standalone website, so you need a ChatGPT account to use the current version.
Readers who forgot a title but remember scenes use Book Detective to match fragmented descriptions against both the LLM and Joelbooks' quality-filtered catalog. Authors can submit books directly to the dataset. The original whatbook URL on Joelbooks now 404s; Book Detective is the documented successor as of May 2024.
Searches books from natural-language descriptions like plot twists, characters, or moods
Combines GPT-4 with a Joelbooks dataset of 10,000+ curated fiction and nonfiction titles
Database updated weekly with fresh and upcoming releases outside ChatGPT's knowledge cutoff
Available as Book Detective in the official OpenAI GPT Store since May 2024
Authors can submit books directly to the searchable AI dataset
Outperformed BookAbout and Findyournextbook in Joelbooks' early comparison tests
Handles fuzzy plot-memory searches better than keyword-only book finders.
Weekly dataset updates include upcoming titles missing from base ChatGPT knowledge.
Authors can submit titles to improve discoverability in search results.
Requires ChatGPT; there is no standalone web app at the old whatbook URL anymore.
Dataset is curated at 10,000+ titles, smaller than full retail catalogs.
Tied to OpenAI's GPT Store, so availability depends on ChatGPT platform policies.
What happened to What is that Books?
What is that Books was the alpha name for Joelbooks' AI book finder. Book Detective replaced it and now runs as a custom GPT in the OpenAI GPT Store. The old whatbook page on Joelbooks returns 404.
How do I use Book Detective?
Book Detective is available in the OpenAI GPT Store. Search for Book Detective inside ChatGPT, then describe the book you remember in plain language. Book Detective matches your prompt against GPT-4 and Joelbooks' curated dataset.
Is Book Detective free?
Book Detective is free to access through ChatGPT's GPT Store. You need a ChatGPT account, and Plus users get the full GPT-4 experience behind the tool.
What data does Book Detective search?
Book Detective uses OpenAI's LLM plus a Joelbooks-maintained dataset of 10,000+ book excerpts and descriptions submitted by authors or written by editors. The team plans to expand toward 100,000 quality-filtered titles.
Can authors add books to Book Detective?
Yes. Joelbooks lets authors submit new or older titles directly to the Book Detective AI dataset so readers can discover them through conversational search.
How is Book Detective different from ChatGPT alone?
Book Detective adds Joelbooks' weekly-updated book dataset on top of GPT-4, including upcoming titles ChatGPT's base knowledge does not cover. Joelbooks reports it finds fresher matches than other book-search GPTs limited to April 2023 data.