These were a series of squarebound mags (we'd probably call them bookazines today) with various gatefold pages inserted between yer more usual vertical A4 (or thereabouts) pages.
The all-new content overlapped with the similar in-universe stuff being pioneered by West End Games, although the Lucasfilm continuity police were presumably keeping things consistant. I'm not expert enough to know for sure.
The third volume - which I'll post soon - covered (surprise) the hardware of the Rebellion. The content of all three were subsequently collected into book form. The UK edition was from Boxtree and I have a battered copy somewhere.
The first edition, published the same year, and featured in a previous post, covered Tatooine.