In the category of "why didn't I read this before?" is a helpful guide that colleagues have told me about, but that I never used until I was looking for a text for the graduate critical methods class I'm teaching this term. I'm referring to the Oxford Guide to Film Studies. I've sometimes overlooked guides of this sort because a) they're really expensive and b) they tend to fall between two stools, not actually a useful introduction for novices and not self-sufficient scholarship either. And indeed, I wouldn't recommend the Oxford guide as an introductory book. But for those who are familiar with film studies scholarship but want a good summary of an area or subfield, this is the book for you. You want learn everything you need to know about semiotics or genre theory or postcoloniality here, but you will have a scholar summing up the field and pointing you to readings, both foundational and more current.
Of course, it's not perfect: Not all the chapters are of equal quality. The focus tends toward the field as constituted in the UK, and some currents (cognitivism, say) are missing. And the pricetag is hefty. But if you're studying for exams, boning up on areas you've long forgotten, or just want a nice bibliographic resource, the guide is excellent. Similarly, undergraduates who have enjoyed their film classes but aren't sure whether grad school is for them - or are just starting to articulate the kind of work they want to do in grad school - will find value in the summaries contained in this book.