Here are three essential little works, all translated by Srila Prabhupada and quoted by him extensively in his books and lectures.
Siksastaka consists of eight verses on the chanting of the Holy Name by Lord Caitanya Himself. Their importance for every Vaishnava cannot be overestimated.
Upadesamrta consists of eleven instructions (the title translates as "The Nectar of Instruction") concerning the process of devotional service to Krsna.
Isopanisad, the foremost of the Upanisads, is one of Prabhupada's first translated works to appear in America.