I am looking for borrowable books in English about Word for Windows 3.1.

I assume you want to use Word for Windows word processor software with Windows 3.1 operating system and that you are using the version 6 or earlier of Word for Windows, so the proper guidebooks should cover these earlier versions of Word for Windows:

Word for Windows 1.x,
Word for Windows 2.x and
Word 6 for windows.

Notice that there isn't any version 3.1 of Word for Windows, whereas windows 3.1 was a major operating system still used by many people including you.

These books are still available in public libraries of the capitol region and match your requirements:

PERSON, RON: USING WORD FOR WINDOWS 2
Lichty, Tom: Desktop publishing with Word for Windows for version 2
Tyson, Herbert: Microsoft Word for Windows revealed
BEASON, PAMELA: MASTERING MICROSOFT WORD FOR WINDOWS
CRANE, MARK: WORD FOR WINDOWS COMPANION
BORLAND, RUSSEL: WORKING WITH WORD FOR WINDOWS
Dustdar, Stephan: Word for Windows powertools
Dean, David: Using Microsoft Word for Windows

You can check the availability of these books by using Plussa, our common bibliographics database in http://www.libplussa.fi/ (also in english) and order them to the nearest library by phone.