Is there a way I can add out-of-print books to my wish list? I'm looking to complete an encyclopedia set that was published in the 1960's and which have no ISBN number?

Trixy.  Without an ISBN, books are posted into PBS with a sytem generated 'short ISBN'.  These are only meant to be used one time, and future books (even if the same title) are supposed to be listed with a new 'short' number.  

If the book you want is already listed under a short ISBN, you can request it.  If any of these temporary PBS numbers already exist in the system for the book you want, you could try wishlisting them...questionable if they would be reused since PBS guidelines discourage that.  Technically, any copies of your book posted to PBS in the future should be listed under a newly issued 'short' number, so you would have to check frequently.

Anyone have better advice?

Books with no #, but have been listed and given a "short" number, can be "wishlisted" by clicking on that title in the listing.  In fact, some of those "no ISBN#" books may be "wished" by several members.  If I want to list one that I've found with no #, I go to the advanced search function and put in the title and author, sometimes the publisher, and see if it's already wishlisted, or, if not, it has been previously listed by another member and just list it under their description.  It saves me a lot of effort that way, and cuts down on the multiplicity of listings for the same title/author, all with no #.    (If some of this didn't make sense, send me a note and I'll try to clarify it.)    Winston

"These are only meant to be used one time, and future books (even if the same title) are supposed to be listed with a new 'short' number. "

Is that really true?

Yes, John, it really is true. The Help Center actually states that. I don't know why, but it does actually state that your book must match the numbers given, and if you have a non-ISBN book, and are trying to post it to a PBS-generated ISBN, then the book in your hand does NOT have that ISBN on it, does it? Therefore the first thing you do to list that book is lie to the system when it asks about your book's ISBN. From the Help Center:

Thus, each "non-ISBN" item represents one copy of the book, and unless you have that exact copy that was posted using that listing by the original poster, the information (eg, book dimensions) about your copy of the book may not apply.

So, it sounds like it's probably not going to work, if I'm understanding the consensus.  Thanks for your responses.

Not necessarily, Rachel. I did receive a non-ISBN book by finding a non-ISBN entry for it and WL'ng it. It's very rare, and the person would have to search for it by the exact same name as your listing had it, but it can happen. I received a cookbook because someone posted one that I was interested in, and I thought, "well this will never do any good, but I'll try it anyway" about a year or so later, someone did indeed post to that listing. She sent me a PM after I accepted and made sure we were talking about the exact same book, and we were and I was delighted. So, it does happen, just not routinely.