Signed Books - How to find them
Signed books are popular with collectors for a number of reasons.
The most important is probably the link with whoever has signed the book. Usually it is the author, but occasionally it may be the illustrator or publisher, or even a previous owner. But for the collector, the signature is proof that the signer has handled the very book which they have in their possession. Such is the power of material possessions that this link can make a book much more desirable.
This leads on to the next factor with signed books. Because they are more desirable, they become more valuable. This of course depends on the fame or notoriety of the signer. As a general principle, dead signers are more sought after than living ones (because they can't sign any more I suppose), and famous signers are more popular than obscure ones for obvious reasons. Other than that, there is not much guidance to give.
For example, many years ago I took a copy of Edward Heath's book on sailing to a nautical bookshop. It was signed by our illustrious ex-premier, and I hoped it might be worth a few pounds to make my trip worthwhile. The amused bookseller remarked that there were actually more signed copies of the book available than unsigned ones, as Ted had promoted his book with great gusto, thereby, in theory, making the unsigned copies rarer and therefore of more value to collectors. In fact the book was, in bookselling terms, a 'dog' anyway, and of little value signed or unsigned.
Roll forward ten years, and I picked up a limited edition of Margaret Thatcher's autobiography, which commanded a very good price. It seems some authors have the golden touch where others do not, and this seems to be down to the charisma of the author. For example, amongst politicians, Churchill and Thatcher carry much weight, while Callaghan and Heath carry less. Film stars and pop stars also add value when they sign items, although this is often short-lived until the next big thing comes along. What price a Bay City Rollers autograph these days I wonder?
All in all, go for what you like personally would be my advice. If buying for investment, go for the writers with proven longevity that is likely to stay that way; star politicans like Thatcher and Churchill, or big authors like Orwell or Nabokov. Good hunting.