Direct answer: Post the plot details you remember (setting, rough time period, any character names, and how it ends) to Reddit's r/tipofmytongue or r/whatsthatbook, and search Google using distinctive plot phrases in quotation marks — between the two, most half-remembered books get identified within a day.
What details actually help
Specific, unusual details matter far more than genre. A dragon or a love story narrows almost nothing; a scene where a character loses a finger in a specific way, or a distinctive object central to the plot, is what actually gets a book identified.
Where to post your description
Reddit's dedicated identification communities have large, active memberships specifically built around solving exactly this problem, and often respond within hours. Goodreads also has a long-running "What's the Name of That Book?" discussion group with a searchable archive of thousands of solved cases.
Using search engines directly
Try searching a very specific, unusual phrase or scene description in quotation marks — this works surprisingly often for books discussed in reviews or summaries online, especially if you remember distinctive wording rather than paraphrasing it yourself.
Librarians as a resource
Reference librarians do this kind of identification professionally and have access to specialized plot-search databases (like NoveList) not available to the general public — a real, underused option for a stubborn case.
What if I only remember the cover, not the plot?
Try a reverse image search if you have any photo, even an old one, or describe the cover's colors and imagery in a plot-identification forum post.
How long does it usually take to get an answer?
Popular or recently-read books are often identified within hours on Reddit; obscure or older titles can take days or longer.
Are there tools built specifically for this?
NoveList (available through many library accounts) allows plot- and theme-based searching, which is more precise than a general search engine for this purpose.