On page 111 of the 5E hand book the invocations Misty Visions, Mask of Many Faces, Master of Myriad Forms, and Otherworldly Leap all they that you can cast them without expending a spell slot. Does this mean the spells essentially become cantrips and I can cast them as often as I want?
YES. Basically, the idea of Warlock's Eldritch Invocations is that their knowledge allows them to gain certain spells, like alter self and disguise self, and for them to act as cantrips. Don't forget however that a lot of these require prerequisites anyway, like levels, and some require rests as well between uses..