Most of what I know I have learned by solving practical problems and by trying to help other Excel users in groups like this one. One important thing in understanding complex formulas is to master Excel's Evaluate Formula tool/F9 technique.

There are almost an infinite number of websites with Excel stuff. If you are specifically interested in array formulas you might want to take a look at:

https://www.ablebits.com/office-addi...nctions-excel/

http://www.emailoffice.com/excel/arrays-bobumlas.html

The last one is superb but really advanced.