According to a sales description, the Harte quote on Twain appeared in:

THE LIFE OF BRET HARTE; by T. Edgar Pemberton; C. Arthur Pearson Limited;
London, 1903, 358 pages. Rare standard life of Harte printed by
Ballantyne, Hanson and Co, Edinburgh and London. This book includes
Harte's well-known description of Mark Twain: "He had the curly hair, the
aquiline nose, and even the aquiline eye--an eye so eagle-like, that a
second lid would not have surprised me..."

I've not examined the book nor found an earlier reference to the quote
published elsewhere.

Barb