This is my favorite introduction to IFS, but it's also a genuinely comprehensive resource. I imagine one can use it over and over and it's enough to support a deep practice. Half of the chapters are guided IFS practices and half are talking about IFS.
Richard Schwartz, its author and IFS creator, recommended this as the best introduction to IFS. Available only as an audiobook.
https://www.audible.com/pd/Greater-Than-the-Sum-of-Our-Parts-Audiobook/B07D4P2BN8