initial-letter doesn't show the correct font

Question

I'm creating an ebook with calibre but I can't set a good drop cap because the letter doesn't follow the font-family rule in the css.

Here's my code:

p:first-of-type:first-letter {
    font-family: "Dearest" !important;
    initial-letter: 3;
    -webkit-initial-letter: 3;
    margin-right: 0.5em;
    color: #903;
}

The drop cap works only with the font specified in p { } (I tested the ebook on iBooks for Mac) How can I solve this issue?