The most authentic source for Hindu scriptural texts is the or the Sundaresan collections . These often host scanned PDF copies of the book titled "Narayaneeyam with Tamil Translation" published by reputable presses like the Guruvayur Devaswom.
(Most Reliable)
ஸாந்த்ராநந்தாவபோதாத்மகம்...
Description of the Lord's form, the glory of Bhakti, and the creation of the world.