Yes, LATEST is a Scrabble word!

Letter values in Scrabble US: L 1, A 1, T 1, E 1, S 1, T 1.

Is it a valid word?

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

up to the immediate present; most recent or most up-to-date
in the current fashion or style
being or occurring at an advanced period of time or after a usual or expected time
after the expected or usual time; delayed
of the immediate past or just previous to the present time
having died recently
of a later stage in the development of a language or literature; used especially of dead languages
at or toward an end or late period or stage of development
(used especially of persons) of the immediate past

Also: up-to-the-minute, a la mode, in style, in vogue.