I appreciated the notes provided in the Pevear and Volokhonsky version of The Brothers Karamazov, which provided a lot of insight missing from my barebones Garnett paperback. P&V have attracted some criticism for succeeding more on hype than quality. For a casual reader like me, I am not sure it makes much difference.
Is yours a library copy? I wouldn't recommend reading a 1390-page book on mobile, but Project Gutenberg does offer a "read this book online" link.