Hi,
I'm new to Calibre, only started using it a few weeks ago and spent almost as much time configuring the viewer as reading

I've encountered a strange problem and manage to solve it, so i thought i'd share the solution and possibly point out a problem.
I'm using the built in viewer to read books, and everything was peachy until i opened a book that wouldn't show ligatures. Spent some time trying to find a solution here, but had no luck. Then tried copying and pasting part of the text from the book to a text file, converted it to epub and ligatures were there, so i knew that the problem was in the ebook itself.
Turns out that the problem was in the stylesheets: i opened the book in Sigil and played with the stylesheets until i found out that deleting all the lines that say
font-face: "Times New Roman";
fixed the problem.
So, the viewer is configured to use "Adobe Caslon Pro." When the book's stylesheet says it should use Times, the text is displayed in Caslon, but the ligatures are missing. When the book doesn't say anything about the font face, viewer is still using Caslon, but the ligatures are working.