New: Improve match by adding small penalty for missing isbn/asin

src/NzbDrone.Core/MediaFiles/BookImport/Identification/Distance.cs

@@ -14,7 +14,9 @@ namespace NzbDrone.Core.MediaFiles.BookImport.Identification
             { "author", 3.0 },
             { "book", 3.0 },
             { "isbn", 10.0 },
+            { "isbn_missing", 0.1 },
             { "asin", 10.0 },
+            { "asin_missing", 0.1 },
             { "media_count", 1.0 },
             { "media_format", 1.0 },
             { "year", 1.0 },

src/NzbDrone.Core/MediaFiles/BookImport/Identification/DistanceCalculator.cs

@@ -81,6 +81,11 @@ namespace NzbDrone.Core.MediaFiles.BookImport.Identification
                 dist.AddBool("isbn", isbn != edition.Isbn13);
                 Logger.Trace("isbn: '{0}' vs '{1}'; {2}", isbn, edition.Isbn13, dist.NormalizedDistance());
             }
+            else if (isbn.IsNullOrWhiteSpace() != edition.Isbn13.IsNullOrWhiteSpace())
+            {
+                dist.AddBool("isbn_missing", true);
+                Logger.Trace("isbn: '{0}' vs '{1}'; {2}", isbn, edition.Isbn13, dist.NormalizedDistance());
+            }
 
             var asin = localTracks.MostCommon(x => x.FileTrackInfo.Asin);
             if (asin.IsNotNullOrWhiteSpace() && edition.Asin.IsNotNullOrWhiteSpace())
@@ -88,6 +93,11 @@ namespace NzbDrone.Core.MediaFiles.BookImport.Identification
                 dist.AddBool("asin", asin != edition.Asin);
                 Logger.Trace("asin: '{0}' vs '{1}'; {2}", asin, edition.Asin, dist.NormalizedDistance());
             }
+            else if (asin.IsNullOrWhiteSpace() != edition.Asin.IsNullOrWhiteSpace())
+            {
+                dist.AddBool("asin_missing", true);
+                Logger.Trace("asin: '{0}' vs '{1}'; {2}", asin, edition.Asin, dist.NormalizedDistance());
+            }
 
             // Year
             var localYear = localTracks.MostCommon(x => x.FileTrackInfo.Year);