Which authentic Hindu scripture contains the total story of Lord Venkateswara?
From where one can read it?
The Skanda Purana has a lot about Venkateswara.
From this chapter of the book "Essence of Skanda Purana" (hosted @ Kamakoti.org):
Sage Suta prescribed an eight-fold Bhakti Route to Lord Venkateswara: 1)To display and ensure ‘Sneha Bhava’(feeling of friendship) towards His Bhaktas (devotees); 2) To perform service to ‘Bhagavat Bhaktas’and satisfy them; 3) To perform Puja to Bhagawan with sincerity; 4) All the actions of the Self are to be targetted to Bhagavan; ..... Those who are fortunate to have even a glimpse of the fantastic Idol of Venkateswara do experience the awe and satisfaction that leaves behind a lasting familiarity and nearness.Those who think of Srinivasa, either out of curiosity, or greediness, love of materialism, or fear or casual occurrence undergo a definite transformation of outlook, approach to life and morality. If such casual approach has a multiplier effect, could one imagine the far reaching impact of the Eight-folded dedication!
And, from this chapter of the same book:
Varaha Deva described to Bhu Devi that Emperor Akash decided to perform a Grand Yagna on the banks of Arani and while tilling, he found a girl child and as he was issueless, brought her up as their own, named as Padmavati. Once Sage Narad met Padmavati as she was grown up as an eligible bride and predicted that she would be the wife of Lord Srinisasa of Venkatachal. Almost immediately she met a Stranger in the Royal Garden and introduced himself as Anant or Veerapati from Venkatachal Mountain and that he was hunting a wolf. Padmavati’s companions rebuffed Ananta Deva that hunting was prohibited in their garden. Anantha returned but desired intensely that Padmavati should become His life partner and sent Vakulamala to the court of Emperor Aakash to negotiate their wedding. The Emperor had no bounds of joy and the holy wedding of Lord Venkateswara and Padmavati took place in style and grandeur when Devi Lakshmi was present to make Padmavati happy and comfortable,