I think what is meant is that “i” becomes “in” when used before the title of a book, etc. In the case you refer to, you would have to write “In “I Lár an tSamhraidh” a léigh mé é”.
Sorry! 🙂 Then it should be “in “Lár an tSamhraidh” a léigh mé é”, etc.
This is a handy rule, as it dispenses you from having to use eclipsis. Otherwise, you would have to say “I gCré na Cille a léigh mé é seo” (“I read it while buried in churchyard clay”), whereas what you mean is “In Cré na Cille a léigh mé seo” (“I read it in a book titled Cré na Cille”).
Makes sense when you put it that way! It’s funny – there are so many rules like this in Irish that I am sure I will still be encountering them years from now.