श्रीमद भगवद्गीता यथा रूप - श्री श्रीमद् ए. सी. भक्तिवेदांत स्वामी प्रभुपाद, संस्थापक‌ आचार्य : अंतराष्ट्रीय कृष्णभावनामृत संघ (इस्कॉन)

Why do we consider Krishna as God ?

It's from the book 'Shri Krishna', i.e. a summary of Tenth Canto of Shrimad Bhagavat Purana, an eternal or Sanatan scripture written by Sage Ved Vyas, literary incarnation of Lord Vishnu as per Vishnu Sahasranama. It also contains the most widely read worldwide 'Bhagavad Gita "As It Is"' spoken by God or Lord Krishna himself without any interpretation of commentator or author. 
The books are authored by His Divine Grace A.C. Bhaktivedanta Swami Srila Prabhupada, founder Acharya of ISKCON, International Society for Krishna Consciousness. You can purchase books online and at any ISKCON Centre.