Try https://gumroad.com/l/RKxO
I purchased this database about 3 weeks ago for a book citation app I'm making. I haven't had any quality problems and virtually any book I scanned was found. The only problem is that they provide the file in CSV and I had to convert 20 million lines which took me almost an hour! Also, the monthly updates are not delta and the entire database is sent which works for me but might be some work for others.
