Crisis on Multiple Earths collections are conveniently available wherever comic books are sold collecting the original multiverse adventures of the Justice League/Justice Society of America. Experience the multiverse as it was originally presented before Crisis on Infinite Earths.
Reading list as follows
CRISIS ON MULTIPLE EARTHS: THE TEAM UPS Vol 1 kicks off with the very first groundbreaking adventure into a parrallel earth with "The Flash of Two Worlds." Here Barry Allen meets his counterpart from Earth 2,Jay Garrick and the muliverse is set into existence in the DC universe. These issues are also available in the DC Classic Library collection but are a great companion to the other crossovers in this edition. The issues include THE FLASH #123, 129, 137, 151, GREEN LANTERN #40 SHOWCASE #55-56, and THE BRAVE AND THE BOLD #61. SHOWCASE #55-56 and THE BRAVE AND THE BOLD #61 aren't essential to the Earth 1 universe but are nice to see an Earth 2 alone.
CRISIS ON MULTIPLE EARTHS VOL. 1 provides the very first JLA/JSA crossovers as the entire JLA becomes aware of their counterparts and asks for their help in times of crisis. After this they meet every year to fight off a crisis until 1985. Earth-3 (The Crime Syndicate) and Earth-A are also revealed and the appearance of Anti-Matter. This collection includes JUSTICE LEAGUE OF AMERICA #21-22, #29-30, #37-38, #46-47.
In CRISIS ON MULTIPLE EARTHS: THE TEAM-UPS Vol 2, The Flashs, Green Lanterns, and Atoms of 2 worlds team-up leading into more JSA/JLA crossovers. Also see more single JSA team-ups. This collects THE ATOM #29, 36, THE BRAVE AND THE BOLD #62, THE FLASH #170, 173, GREEN LANTERN #45, 52 and THE SPECTRE #3.
CRISIS ON MULTIPLE EARTHS VOL. 2 has more JLA/JSA crossovers as they battle their yearly crises. More Earth-2 counter-parts essential to the future Crises come out in this volume too. This collects JUSTICE LEAGUE OF AMERICA #55-56, 64-65, 72-73, 83-84.
CRISIS ON MULTIPLE EARTHS Vol 2 collects more of the JLA/JSA yearly crises. In this volume the Freedom Fighters of Earth-X make their Multiversal apperance and the Seven Soldiers of Victory show their places in the multiverse collecting JUSTICE LEAGUE OF AMERICA #91-92, #100-102, #107-108, #113.
CRISIS ON MULTIPLE EARTHS Vol 4 collects even more JLA/JSA team-ups as they battle crises like Mordru. This volume also collects where the Captain Marvel Shazam family of Earth-S stand in the multiverse for the first time, and meeting their counterparts.
JUSTICE SOCIETY OF AMERICA Vol 1 collects some of the solo adventures of the Earth-2 JSA in All Star Comics.
CRISIS ON MULTIPLE EARTHS Vol 5 collects more JLA/JSA adventures as they find the Five Warriors From Forever and face the evil god, Darkseid. This collects JUSTICE LEAGUE OF AMERICA #159, 160, 171, 172, 183, 184 and 185.
Will there be a Crisis on Multiple Earths Vol 6? There are still more JLA/JSA team-ups uncollected. Until then we can try to find these issues at comic shops and coventions.
JUSTICE LEAGUE OF AMERICA #195-197, #207-209, #119-120, #231-232.
Leaving JUSTICE LEAGUE OF AMERICA #224 as the last JLA/JSA crossover they team up for the last time in Crisis on Infinite Earths. Them Earths are merged making the JSA and JLA co-exist destroying all alternate versions (excluding The Flash.)