Vital question about adding books

Does anyone - ANYONE! - knows how to find books that are not on any of the official sites but other people have on their bookshelves (added manually)?


Please, help! 
Many thanks :)


Currently reading

Best Kept Lies
Helena Maeve
W.A. Hoffman
Progress: 3 %
The Virtu
Sarah Monette
Progress: 30 %
Orochi no Kishi
Itoshi, Lehanan Aida
Progress: 80 %