I'm a new user and I've only downloaded a couple groups of 10-15 books each but each time I've gotten an error with the trace below on one or two of the books in the group.
Thus far I've hit ignore and nothing obviously terrible has happened although I have not taken a thorough look at the output of AAX Audio Converter to see if all the chapters were actually exported or the chapter data is correct.

Microsoft.Data.Sqlite.SqliteException (0x80004005): SQLite Error 19: 'UNIQUE constraint failed: ChapterInfos.Id'.
at Microsoft.Data.Sqlite.SqliteException.ThrowExceptionForRC(Int32 rc, sqlite3 db)
at Microsoft.Data.Sqlite.SqliteDataReader.NextResult()
at Microsoft.Data.Sqlite.SqliteCommand.ExecuteReader(CommandBehavior behavior)
at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReader(RelationalCommandParameterObject parameterObject)
at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.Execute(IRelationalConnection connection)