Read first, or watch first?
Read first if you can. The engine of this book is a question — where is Father? — and the film answers it on your face in the first twenty minutes; the platform scene at the end cannot be unspoiled once seen. There's also a practical wrinkle in the US: the 1970 film, which is the one to want, has no clean American release, while the two easiest to find on Amazon are the 2000 Masterpiece version and a 2022 sequel that isn't this story at all. So book first is both the better order and the simpler one. When you do track down Jeffries' film, watch it together — it is faithful enough that it plays as a reunion rather than a correction.