It is obvious that I like British Romantic literature, so I'll cheat and name anything but.
Useful? Like The Little Brown Handbook or Grammar To Go? Useful isn't usually what I go for (And what could be more exciting than grammar? I'm being serious!)
A few texts that I enjoy teaching that students tend to enjoy too:
William Hunter's obstetrical atlas (yes, it's called an atlas) Anatomy of the Human Gravid Uterus from 1774 doesn't sound like fun, but... the very idea of a sumptuously engraved book of dead pregnant women cut open is fascinating to me. It is strangely aesthetic: one of the illustrations shows a reflection, in the glistening flesh, of the light from the window. It is an enormous tome (I think you'd call it an elephant folio), and the British Library copy is crumbling at the spine, so it makes for a wonderful experience of the materiality of books: the heft of it, and having to stand up to turn the pages; the smell of it, and the dusty mould getting all over my hands and my clothes. Not getting my hands dirty quite the same way the anatomists did...
Looking at, and even touching, original Blakes -- particularly the King's College copy of Songs of Innocence and of Experience, with its gorgeous, rich colouring, complete with gold leaf -- that was okay!
I'm currently working on an article about Blake and Jim Morrison. Don't think I need to explain why that's fun.
A rock star! Or a poet. Being an English professor was my practical option (ha!)
Click here to view an image of Dr. Connolly on the Faculty of Arts 50th Anniversary website.