The best commentaries, translations of 700 shlokas verses of Bhagavad Gita containing original Sanskrit text along with Hindi and English transliteration is available for mere $1 from Gita press, Gorakhpur, India.
Gita press has commentaries on Bhagavad Gita both in Hindi and English by revered Acharyas like Jayadayal Goyandka, Swami Ramsukhdas and Hanuman Prasad Poddar.
Commentaries on Bhagavad Gita and Upanishads (independent spiritual treatises) by Adi Shankaracharya (an enlightened one) are also available and very popular.
Gita press has online presence and books can be ordered online.
Furthermore, we must always bear one thing in mind; the gist of Bhagavad Gita can only be understood indulging in contemplation (chintan), never literally.