it depends on which branches of maths you're interested in. I find brilliant an amazing app to try out the different math branches, see which ones resonate with you and the way they explain things is really well done. for example: I read Euclid's proof of the infinitude of prime numbers maybe 20 times in a book and I just couldn't wrap my head around. Brilliant just let me do the math myself and then asked if that means there are infinitely many prime numbers and I was like: Oh my god, yes, it does! of course ![Smile :)](./images/smilies/icon_smile.gif)
for advanced maths I can recommend the following books which I am happy to send as pdf's to whomever is interested:
Sheldon Adler: Linear Algebra done right. (2015)
Lewinter, M.: Elementary Number Theory with Programming. 2ed. (2008)
Crawford, M.: Introduction into Number Theory. (2008)
Borden, B. & Luscombe, J.: Mathematical Methods in Physics,Engineering, and Chemistry. (2020)
McMullen, C.: Essential Calculus Skills. Practice Workbook with Full Solutions. (2018)
Tao, T. Analysis II. 4th ed. (2023)
Chartrand, G. et al.: Mathematical Proofs: A Transition to Advanced Mathematics. (2018).