1914

Battles of Gumbinnen, Tannenberg and 1st Masurian Lakes in East Prussia. Battles of Rawa Ruska, Lipanow Limanowa, Lemberg and Krakow in Galicia, and the Sieges of Przemysl (1st and 2nd). Battles of the Vistula and Lodz in Poland.

1915

The 'Black and Gold' Offensives in the Carpathians, 2nd Battle of the Masurian Lakes in East Prussia, the Gorlice-Tarnow Offensive in Poland, the German Baltic Offensive, and the Great Retreat.

1916

Lake Naroch Offensive, the Brusilov Offensive, the Battle of Kowel, the Romanian Invasion of Transylvania, the Invasion of Romania.

1917

The Kerensky Offensive, Ternopol Counter-Offensive, Battles of Marasti and Marasesti, Riga Offensive, Operation Albion.

After that, the only really major effort in 1918 was Operation Faustschlag, aimed at enforcing the Brest-Litovsk Treaty, and that faced little resistance. If you want good books, Prit Buttar has written a trilogy for Osprey Publishing on the Eastern Front, starting with Collision of Empires and followed by Germany Ascendant, with the third volume on Russia in 1916-17 coming out in the near future. Norman Stone's The Eastern Front is a classic, although even he admits that by now it's pretty dated. Timothy Dowling has written a good book about the Brusilov Offensive, Richard L. Dinardo has written a book about Gorlice-Tarnow and Graydon Tunstall about the Carpathian front in 1915. Glenn Torrey has written about the Romanian Front, Michael Barret more specifically about the Central Powers invasion in 1916. Barret also has a book about Operation Albion, the German landings at the Moonsund Islands in 1917. Vejas Liulevicius' books about occupation on the Eastern Front are also highly commendable: War Land on the Eastern Front and The German Myth of the East.