[Divingduck]

@davidfor,
Thanks for the beta. I run a test against some German EPUB files in different genres. As reference information I made a docx-conversion. Here are the results.

1. A Federal Agency for Civic Education document
(Link:http://www.bpb.de/system/files/datei/APuZ_2015-52.epub)
Count Page/Word Statistics
Word count - old method: 30,349
Word count: 30,539
MS Word 2013 Statistics
Words: 29,864

Spoiler:

Count Page/Word Statistics
Logfile for book ID 15709 (Europäische Integration in der Krise)
Found 30539 words
Computed 11.9 Flesch-Kincaid Grade
Computed 39.7 Flesch Reading
Computed 12.0 Gunning Fog Index
Found 111 pages
15709
InputFormatPlugin: EPUB Input running
on C:\Users\xxxxx\AppData\Local\Temp\calibre_mediha\o zhs7n_count_pages\15709.epub
Found HTML cover OEBPS/00000000000_cover.html
Page count: 111.0
Word count - old method: 30349
Word count: 30539
Results of NLTK text analysis:
Number of characters: 199944
Number of words: 32658
Number of sentences: 1888
Number of syllables: 57866
Number of complex words: 4280
Average words per sentence: 17
Flesch Reading Ease: 39.6790997612
Flesch Kincade Grade: 11.9481633903
Gunning Fog: 12.0422071162
MS Word 2013 Statistics
Characters: 200404
Characters w space: 229956
paragraphs: 693
Words: 29864

2. Dale Brown - Außer Kontrolle (Thriller)
isbn:9783641111038
Count Page/Word Statistics
Word count - old method: 109,218
Word count: 106,689
MS Word 2013 Statistic
Words: 107,033

Spoiler:

Count Page/Word Statistics
Logfile for book ID 15535 (Außer Kontrolle)
Found 106689 words
Computed 9.3 Flesch-Kincaid Grade
Computed 56.7 Flesch Reading
Computed 10.1 Gunning Fog Index
Found 322 pages
15535
InputFormatPlugin: EPUB Input running
on C:\Users\xxxxx\AppData\Local\Temp\calibre_mediha\c lqzsw_count_pages\15535.epub
Found HTML cover titlepage.xhtml
Page count: 322.0
Word count - old method: 109218
Word count: 106689
Results of NLTK text analysis:
Number of characters: 648986
Number of words: 115430
Number of sentences: 6878
Number of syllables: 182710
Number of complex words: 10629
Average words per sentence: 16
Flesch Reading Ease: 56.6846993849
Flesch Kincade Grade: 9.32779606688
Gunning Fog: 10.0832712466
MS Word 2013 Statistic
Characters: 639284
Characters w space: 744117
paragraphs: 2857
Words: 107033

3. Umberto Eco - Die große Zukunft des Buches (non-fiction)
isbn:9783446236165
Count Page/Word Statistics
Word count - old method: 69,745
Word count: 68,944
MS Word 2013 Statistic
Words: 68,916

Spoiler:

Count Page/Word Statistics
Logfile for book ID 14875 (Die große Zukunft des Buches)
Found 68944 words
Computed 8.2 Flesch-Kincaid Grade
Computed 64.4 Flesch Reading
Computed 9.9 Gunning Fog Index
Found 212 pages
14875
InputFormatPlugin: EPUB Input running
on C:\Users\xxxxx\AppData\Local\Temp\calibre_mediha\i 6wxog_count_pages\14875.epub
Page count: 212.0
Word count - old method: 69745
Word count: 68944
Results of NLTK text analysis:
Number of characters: 383108
Number of words: 74867
Number of sentences: 4560
Number of syllables: 111639
Number of complex words: 6615
Average words per sentence: 16
Flesch Reading Ease: 64.4424975623
Flesch Kincade Grade: 8.24573911069
Gunning Fog: 9.93426743425
MS Word 2013 Statistics
Characters: 384109
Characters w space: 452814
paragraphs: 934
Words: 68916

4. Helena Marten - Die Kaffeemeisterin (historical fiction)
isbn:9783641059606
Count Page/Word Statistics
Word count - old method: 149,472
Word count: 148,460
MS Word 2013 Statistics
Words: 148,883

Spoiler:

Count Page/Word Statistics
Logfile for book ID 15637 (Die Kaffeemeisterin)
Found 148460 words
Computed 7.1 Flesch-Kincaid Grade
Computed 70.7 Flesch Reading
Computed 9.4 Gunning Fog Index
Found 454 pages
15637
InputFormatPlugin: EPUB Input running
on C:\Users\xxxxx\AppData\Local\Temp\calibre_mediha\r o4vpd_count_pages\15637.epub
Page count: 454.0
Word count - old method: 149472
Word count: 148460
Results of NLTK text analysis:
Number of characters: 813152
Number of words: 158706
Number of sentences: 10017
Number of syllables: 226847
Number of complex words: 13451
Average words per sentence: 15
Flesch Reading Ease: 70.6866814109
Flesch Kincade Grade: 7.12637304198
Gunning Fog: 9.39016798357
MS Word 2013 Statistics
Characters: 801879
Characters w space: 948783
paragraphs: 3534
Words: 148883