The STacy was cool bit it was a bit of a brick (7kg!!), much akin to the first Macintosh Portable. The ST Book was much nicer: https://en.wikipedia.org/wiki/ST_Book . It was only 1.9kg, similar in weight to a 2010 MacBook Pro 15". Only big drawback was that it lacked a backlight which the STacy strange enough did have. The ST Book did have a fairly long battery life despite running on only 7 AA batteries!
Unfortunately not very many were made of that one. Only 1000-1200 ST Books compared to the 35000 STacys. It was produced during the downfall of Atari. So they are pretty much unobtainium these days.
The STacy has a battery compartment but in the end Atari decided not to offer a battery option because the runtime was so miserable. They improved that massively with the ST Book (but by cutting the backlight, external video interface and floppy drive among others).