previously opened book won't open

[Yvonne Sharples]

 I have no idead what this means but after I updated Calibre software last night 0.8.51, previously opened book would not open. Tracker info below. My O/s is Windows 7 Home
calibre, version 0.8.51
ERROR: Could not open ebook: u'C:\\Users\\Yvonne\\AppData\\Local\\Temp\\calibre_0.8.51_tmp_vvszhq\\ck7an6_ebook_iter\\OEBPS\\Text\\Cover.xhtml' is not in list

Traceback (most recent call last):
  File "site-packages\calibre\gui2\viewer\main.py", line 38, in run
  File "threading.py", line 483, in run
  File "site-packages\calibre\ebooks\oeb\iterator\book.py", line 173, in __enter__
  File "site-packages\calibre\ebooks\oeb\iterator\spine.py", line 103, in create_indexing_data
  File "site-packages\calibre\ebooks\oeb\iterator\spine.py", line 65, in __init__
ValueError: u'C:\\Users\\Yvonne\\AppData\\Local\\Temp\\calibre_0.8.51_tmp_vvszhq\\ck7an6_ebook_iter\\OEBPS\\Text\\Cover.xhtml' is not in list

Thanks (Love, love love Calibre software!)