When making a classic serif typeface, you need to do plenty of research to make sure it's going to be unique enough to stand out against existing designs. Likewise, if you're planning to do what amounts to a redesign of an old historical typeface (which might not exist as a digital font), you need to study it carefully and check a number of sources to make sure that there's a place in the market for it. Useful places to do your research are on the internet, in libraries and, of course, in printed specimen books.
Check out The Elements of Typographic Styles by Robert Bringhurst, The Encyclopedia of Typefaces by Jaspert, Berry & Johnson and A Book of Type and Design by Oldrich Hlavsa. Identifont is also worth visitiing.
