For various free Mathematics books, check the Books (free) page from the Reading Series of pages; do you have Colin Foster’s lovely KS3 Instant Maths Ideas, the brilliant A Level worked examples from Hodder and the Shell Centre books in your collection?
Note that the Reading series includes these Research pages:
How about an interactive comic on the Art and Science of Memory? How to Remember Anything Forever-ish by Nicky Case (October 2018).
Such an important topic – see also Retrieval Practice.
I was very happily distracted by Nicky’s website, there is so much to explore. Try Explorable Explanations, look at these Mathematics explorations for example; try The Birthday Paradox Experiment or The Monty Hall Problem.
Explorable Explanations, Math – Nicky Case