You might want to take a look at Erik Christiansen's The Northern Crusades. It will give you some detail on the battles between the Lithuanians, Novgorodians and Polish against the Teutonic and Livonian knights. For western battles and tactics you could take a swing at Charles Oman's The Art of War in the Middle Ages, though by now it is considered a dated source and inaccurate in many respects. Medieval Warfare: A History by Maurice Keen might also be a good overview, though it is not exactly a scholarly source and is better for just general knowledge.

Some of the other medieval history buffs here could probably give you much better sources, but there's a couple to look up!