Someone got me a life-sized cardboard cut-out of Sondheim as a joke present a few years ago. It mostly lives folded-up in the cupboard, but occasionally serves the purpose of being an extremely terrifying thing to come back to when the lights are off.
If you want some information on music specifically. there's a book called Sondheim on Music which is a written out interview with Sondheim.
Here's the amazon link but you could probably get it at a library: https://www.amazon.com/Sondheim-Music-Minor-Details-Decisions/dp/0810874369
I think you mean that 1977 movie adaptation? You can buy the DVD.
https://www.amazon.com/Little-Night-Music-Elizabeth-Taylor/dp/B00003CWT3