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.