I am downsizing and have many 'books' how and where can I get a charitable tax receipt ?
Exact rules are going to depend on your location, but let me give you some principles.
Getting a tax receipt for anything is a matter of finding a charity who wants your books, and values them enough to give you a receipt for them. That's going to depend on the books. If they are valuable antiques then you will have no problem, but then you would also have no problem finding a dealer who would buy them from you. If you wish a charity to benefit, sell the valuable books to a dealer and give the proceeds to a charity.
If your books are just regular books, a dealer is not the place to go. However many towns do have second hand bookstores that will buy select used books. Thrift stores are almost always willing to receive used books. However except in rare cases they will not buy goods, and not issue receipts. It is usually not worth their time to make the valuation and do the paperwork, and the value of most used books is low enough that it is not worth your time either.
Certain kinds of books, like textbooks, can be donated for use overseas, and you might have more luck with organizations that want them.