There are a small number of Harris' books currently in print, you can buy them on-line through a site such as Amazon.com or simply by ordering them through your local bookseller, which may save on shipping costs. This page concentrates on second-hand (used) books, as that is a more interesting and thornier topic.
When looking for books, the best place to start is probably AddAll.com or Bookfinder. (At the end of this page there is a form that you can use to search AddAll.com). These services search across a number of different sources to find books matching your requirements. Before buying any books, if you are new to book collecting it is worth spending some time familiarising yourself with common bookselling terms.
Another source for books is the online auction, such as eBay. Don't expect always to get a bargain, as many goods end up being sold at well above their normal collector's price when bidders get over-excited. Be self-disciplined, and always make sure you know what you are bidding for, or you may end up paying a lot of money for trash.
First Editions and other traps for the unwary
Identification of a true first edition can be very difficult: I would not even dare to begin to touch on the subject. Different countries have different practices and these have changed over the years. That being said, many of Frank's books were not best-sellers at the time of publication so these are relatively easy to sort out. The bibliography on this site attempts to list all important or interesting editions, but is certainly not complete, nor completely accurate.
The most important piece of advice about first editions I can give is: don't believe what booksellers tell you. I have seen many examples of books being described as first editions when they are dated years too late, sometimes by decades. Sellers on eBay, who are often amateurs, may not be expected to know much about books, but professional bookdealers are often just as bad. Slippery terms, such as 'first thus', are used to exaggerate the value of a book.
Anyone who has visited my house knows that I sell far fewer books than I buy, but I have been on the end of enough transactions to offer a modicum of advice. First, find out from sites such as AddAll or Bookfinder an approximate value for the item. If it is an exceptional item then you may have to use some common sense. For example suppose you have a signed first edition of "My Life and Loves" in fine condition, but there are no other examples listed. You might look at other signed editions of Harris books and see what the signature adds to the value. Be prepared to be disappointed, by the way. Very few books are really valuable and the chances are what you have is not interesting to collectors. (Rarity is one factor in the value of a book, but it is by no means the most important. Many very rare books are worth practically nothing.)
Once you are confident about the value of a book, you can then try to sell it. Selling direct to an honest collector, if you can locate one, is a good way to get a fair price for an item. (Please feel free to ask me first!) Auction services such as eBay often realise exaggerated prices, but equally many items sell for far less than they are worth - or don't sell at all.
Now that so many bookdealers are on the 'net, it is easier to locate those who might be interested in your books. ABE and other similar services will help you track them down. Be careful to describe your books accurately to avoid having them returned to you.