These are a few links to other tetrationrelated websites.

Jonathan Bowers  Array Notation, which provides an alternative extension of hyperoperators to more than 3 parameters. 

Ioannis N. Galidakis  Mathematics, which includes many papers detailing tetration and related functions, such as an extension of the Lambert Wfunction to the quaternions. 

Daniel Geisler  Tetration.org, which includes several Mathematica notebooks, and many pages discussing the dynamical and combinatorial aspects of tetration. 

Robert Munafo  Large Numbers, which lists several hundred named large numbers, and has a few articles which discuss the basics of tetration. 

Neil J. A. Sloane  Encyclopedia of Integer Sequences, which has several dozen sequences directly related to tetration, such as A000258. 

Henryk Trappmann  Tetration Forum, which provides an outlet for people to talk about tetration, and an archive of many, many insightful discussions. 

Eric W. Weisstein, Ioannis N. Galidakis 
MathWorld's Power Tower page, which includes many references, series expansions and complex plots of special cases of tetraiton. 

Various  Wikipedia's Tetration page, which is constantly being updated with recently published information. 
These are all of the references that I have found except possibly those contained within classical popular works, such as (K1). If I do not refer to the work in this website, and the referece can be found in one of the papers below, then it is not included. The format I use diverges from standard formats such as MLA, AMS, and so on, but it is closer to other standards like ISO 8601. MLA citations use authors full name and AMS citations use a number, whereas here I use the first letter of the last name along with a number for uniqueness. I think the idea of dropping an authors first name is biased

Robert M Corless, Gaston H. Gonnet, D. E. G. Hare, David J. Jeffrey 
Lambert's W function in Maple, Maple Technical Newsletter, 0 9 (1993), 1222. Available: <here>. 

Robert M Corless, Gaston H. Gonnet, D. E. G. Hare, David J. Jeffrey, D. E. Knuth 
On the Lambert W function, Advances in Computational Mathematics, 5 1 (1996), 329359. Available: <here> Buy: <here>. 

Clément Frappier  Iterations of a kind of exponentials, Fibonacci Quarterly, 29 4 (1991). 

Morgan Ward, Francis Brock Fuller 
The continuous iteration of real functions, Bulletin American Mathematical Society, 42 (1936), 393396. Available: 20071120 <here> or <here>. 

Robert Arther Knoebel  Exponentials reiterated, The American Mathematical Monthly, 88 4 (198104), 235252. Preview: 20071120 <here>. 