I'm a Software Developer. I'd like to learn more about Cluster Analysis because I think it would be an extremely useful skill-set to incorporate into the products my company develops and supports.
I have seven years' experience as a Software Developer. While I generally consider myself mathematically literate, my current level of mathematical ability is that of a first-year university undergraduate... And I graduated back in 2007, so have likely forgotten a few things. So I expect that I have some catching up to do.
I'm fully prepared to pay for books and resources if they come highly recommended.
The problem right now is that I don't know enough to be able to accurately evaluate what is or isn't a good resource.
Any pointers in the right direction would be greatly appreciated.
With your background, maybe you should start with a broader view than only looking for cluster analysis. One good book is Hastie, Tibshirani & Friedman: "The elements of statistical learning", which do include cluster analysis, but within a wider framework.
http://statweb.stanford.edu/~tibs/ElemStatLearn/
If the maths of that book is to tough going, you can try the simpler book with some of the same authors, and associated online MOOC course: https://lagunita.stanford.edu/courses/HumanitiesandScience/StatLearning/Winter2015/about
A more traditional book (with software implemented in R), with emphasis on robust methods is Kaufman, Rousseeuw: "Finding groups in data: An introduction to cluster analysis" which includes well-tested methods. (Wiley)