Yes, EIGHTVOS is a Scrabble word!

Letter values in Scrabble US: E 1, I 1, G 2, H 4, T 1, V 4, O 1, S 1.

Is it a valid word?

EIGHTVOS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the size of a book whose pages are made by folding a sheet of paper three times to form eight leaves

Also: 8vo.