Warthur
It's not glam metal - that style hadn't yet come about - but the third Alice Cooper album has a good claim to being one of the founding documents of glam *rock*. Marc Bolan had gone electric and was still doing fairytale folk, so the album precedes T.Rex's seminal Electric Warrior, whilst David Bowie was still putting together Hunky Dory at this point in time, so the album came out well before the British side of glam really coalesced.
And it's certainly the first manifestation of a particularly American brand of glam rock which Kiss would also make important contributions to - trashy and self-consciously so, controversial in the way it marketed songs with highly sexualised or violent content to teens (even though really all the songs were doing were acknowledging the drives and desires of the teen audience itself), and above all playing some down and dirty rock and roll. Album highlights include the teen angst anthem I'm Eighteen and the concert staple The Ballad of Dwight Fry. It wouldn't be the band's best, but it's the album that put them on their map, and is regarded by many fans as their "true" debut, the two previous albums containing decidedly inferior material.
Cooper still plays a lot of material from this album in concert today - most of his gigs will at least include I'm Eighteen and The Ballad of Dwight Fry - and given his expansive back catalogue that's probably the best recommendation for it.