it’s been a while since I’ve been a beginner so I might not have the best resources, but I would recommend Harvard’s Stat 110 with Joe Blitzstein (lectures online) and then Machine Learning by Kevin Murphy. might be a scarier book to someone not confident in their math, but overall a better one imo

for something more directly comparable to the niche ISLR filled, Bishop’s books are generally better - although I can’t recall their title