"QBasic by Example, Special Edition", a book of about 620 pages.
An excellent book. It ties for first place with "Microsoft QuickBasic Bible" in my estimation. They compliment each other very well. In fact, "QBasic by Example" has a dedicated chapter on variable scope while you have to dig into the index in "Microsoft QuickBasic Bible" to find "scope" under "Variables" which references three separate locations. It's mentioned almost in passing in various sections of the book. So, between these two, you've got a solid reference library.
If you need to level down to newbie status, Greg Perry also wrote "Crash Course in QBasic". It's 272 pages, starts at square one with starting the program, covers all the major functions (except integration of other languages of course), and then ends with a chapter on debugging.
:@)