Can anyone offer any advice on when I should make ++ into a ligature (outside of code examples) and when I shouldn't?

The C++ standard, Incorporating Technical Corrigendum No. 1 [Wiley] (ISBN 0-470-84674-7) always shows ++ as a ligature in the string C++, except in titles where there is a clear gap between the two plus signs.

Another respected C++ reference book, Exceptional C++ [Addison-Wesley] (ISBN 0-201-61562-2) does not show C++ as a ligature, except in the title for the entire series on the front cover — it's not even a ligature in the series title on the back cover!

I am trying to typeset a book (in Linux) in which I want to use the phrase C++ in the book title, chapter titles, and the text of the book. Anyone have any ideas?

Perhaps a better reference than the books you mention would be The C++ Programming Language, written by the inventor.
As far as I can tell (from Stroustrup's faq) the name "C++" does not have any spaces as the "++" is inspired by the post-increment operator.

…but the screenshots on that link don't only show ++ as a ligature; there is a clear gap between the two + signs in some places. A ligature is when two letters are run together (like this character, if your character-set will cope with it: æ).

Here's a larger form of the book cover scan on your link:
http://www.amazon.com/gp/reader/0201...724#readerpage

You will note that the big 3D title puts a gap between the two + signs, while “The Creator of C++” text at the bottom does not. The former is not a ligature, the latter is.

And the second:
http://www.amazon.com/gp/reader/0201...724#readerpage

This doesn't show anything as a ligature on the title page. Going through the table of contents on Amazon, it looks to me like part titles are shown as ligatures while chapter titles aren't.

I'm still confused.

My : Since it is "based" on the increment operator, and C++ programmers seldom (if ever) use a ligature in their code, using it in a book seems overly pedantic, and, basically, wrong.

Thank you! I like that viewpoint.

I suppose the counter-argument would be that C++ code is normally written in a monospaced font, in which ligatures are never used (except perhaps as a special character), whereas books are written in variable-width fonts that do (e.g. “fi” is usually shown as a ligature in words with no dot on the “i”).

I'll leave it unligatured for now unless someone can come up with a reason to do otherwise.