The UML: A little more than boxes and lines
A number of years ago (quite a few now) I wrote myself an abridged guide to UML. I found Krutchen’s “4+1 Architectural View Model” useful. The UML maps it’s diagrams onto this model reasonably well.
I make my guide available as a collection of pages here. This collection of pages present an abridged combination of The UML User Guide and The UML Reference Manual books, taking an 80-20 approach to The UML (80% of designs can be described using 20% of the UML). Refer to the published books for an in-depth understanding of The UML.
I still use (somewhat loosely) about 20% of The UML to document systems - mostly sequence diagrams and component diagrams.
I use a mixture of Krutchen’s 4+1 Architecture View Model and Rozanski & Woods’ Software Systems Architecture for viewpoints and perspectives - along with the C4 model to identify levels of abstraction.
You may find this UML collection useful. I hope you do.