The earliest work of which I am aware is:
G. Linden J. Jacobi and E. Benson, Collaborative Recommendations Using Item-to-Item Similarity Mappings, US Patent 6,266,649 (to Amazon.com), Patent and Trademark Office, Washington, D.C., 2001That patent was filed in 1998 and issued in 2001.
A later academic paper
Greg Linden, Brent Smith, Jeremy York, Amazon.com Recommendations: Item-to-Item Collaborative Filtering, IEEE Internet Computing, v.7 n.1, p. 76-80, January 2003is a more friendly description of the work in the 1998 patent. It cites the patent as previous work.
Another paper that appears to be frequently cited is:
Badrul Sarwar, George Karypis, Joseph Konstan, John Reidl, Item-based collaborative filtering recommendation algorithms, Proceedings of the 10th international conference on World Wide Web, p.285-295, May 01-05, 2001, Hong Kong, Hong KongSome publications mistakenly have written that Sarwar et al. first "introduced" or "proposed" item-to-item collaborative filtering. Even The Economist got this wrong, later issuing a correction.
This confusion may be because the Sarwar et al. paper did not reference the patent. The 1998 patent preceded Sarwar et al. by three years and was public information well before the Sarwar et al. paper was published. The 1998 work probably should have been cited by Sarwar et al., certainly if any of the authors had reviewed it, but probably even if they had not.
I realize the situation is complicated. The 1998 patent usually is referenced with its 2001 issue date, as is the convention, making it less clear that it preceded Sarwar et al. by three years. The Sarwar et al., the first academic publication on the algorithm, does not reference the patent at all, further confusing the issue.
Nevertheless, please be careful about crediting the earliest work. Please note the earlier publications when writing about item-to-item collaborative filtering.