That's a very useful post, Sid, and I hope everyone makes note of it. When the researcher has gone to those locations and eventually identified a book he or she wants to buy, they can then go to the following web site that has a data base of approximately 100-million separate titles and the identity of the bookdealers who have them for sale:
This data base incorporates all of the other on-line book data bases such as ABA Books, Alibris, Campusi, AddALL and many, many others. I have never looked for a book and not found it there. Further, the books are usually accompanied by a descriptive blurb that gives additional information not found in the national library data bases.
Quite often the researcher would prefer to borrow the book rather than purchase it. and there is a special data base for that, too. It's called WorldCat (short for World Catalog). It's the former OCLC data base and has about 70,000,000 titles. Unfortunately, you must go to your local public or university library to use it, since a connection to it is by subscription. Typically, a public library system with a main library and 12 to 18 branch libraries would pay $5,000 a year subscription fees. But WorldCat is worth it because it identifies every single library in the world that owns that book. The researcher can then either go to that library if within travel distance or request it through interlibrary loan.