I found a problem which concerns probably someone else.
I needed to make menu items which are pointing to specific booklibrary category, so I done that with workaround using EXTERNAL LINK menu item. All went well, but when I follow the "external link" which is pointing to some of the booklibrary's categories the Alphabetic index below the pages isn't working, when you select a letter it shows empty list, no books at all even though there are book with that first letter.
I found out that the letter index works fine with latin symbols ( A,B,C,D....X,Y,Z), even with the external link menu item, but when the book title is in Cyrillic - doesn't work, but only when the menu item is linked to an "external link".
When I set a menu item pointing to let's say all Booklibrary's books and hit a cyrillic symbol (ex. А,Б,В,Г,Д....Ж,З,Ю...) it does filter the books!
The problem is not just with external link!!!!
I just switch the library menu item from Show All Books to Show All Categories and when I navigate into the category with cyrillic symbols and tried to filter the books by letter - it doesn't work either.