No, but: 1) it’s good to have Source available for any sections that you struggle with. You may find that a different presentation of the same material clicks better. 2) be aware that the page counts in Mahler do not match exam weight very well. Make sure you spend enough time with the highly weighted topics and don’t waste too much time learning stuff you can basically take from the formula sheet. 3) ISLR is very well-written, relatively short, and the pdf is free, so there’s very little drawbacks to read at least that one