I think you may need to explain 'sight singing'. Would it be the same as 'sight reading'? Singing or playing a new piece without having seen it or being familiar with it beforehand?

If that's the case, then I'd recommend some of the books that EFDSS have published via Cecil Sharp House in the UK. 'Marrowbones' is one that springs to mind, as I have it downstairs. It's clear, easy to follow and the staves are of a size that you don't have to squint at them to determine whether it's a B or a C.

If you already have some musical learning, then it's a great resource. If you're not a dot reader, then it's clear enough (melody lines only) to see where the tune goes. I'm pretty sure the first verse is printed under the staves so you can see how it should fit.

Good luck.

LTS