The RSGB course book for Foundation is called the “Foundation Licence Manual” – Whilst the book is helpful, it’s not essential – as our free course covers the entire syllabus. You don’t need the book, but you may find it helpful.
For more on the book, see: www.essexham.co.uk/train/foundation-licence-manual/
As an alternative, why not try our Foundation Study Guide to support your studying?
It’s available in Paper version or Kindle Version from the Amazon website. It’s also available on Apple Books, Google Books, Kobo and now as an Audible Audiobook and Apple Audiobook