In my personal opinion some interesting links:

 

* understanding Solar-Terrestrial Data and their impact on HF-propagation:

https://www.arrl.org/files/file/Technology/tis/info/pdf/0209038.pdf

 

* L.B. Cebik website with tons of information for radio amateurs:

https://www.antentop.org/w4rnl.001/radio.html

 

* how to build your own bandpass filters:

https://kitsandparts.com/W3NQN_May_June_1998_QST.pdf

 

* how to set ALC correctly in SSB mode:

https://www.hamradioandvision.com/how-to-use-an-alc-meter