The book is primarily a collection of the spiritual "outpourings" or emanations attributed to Shaikh ‘Abd al-Qadir al-Jilani
Be cautious of another work with a similar title:
You can find historical scans and uploaded copies of the book, such as the digitized record from the Digital Library of India on Internet Archive . Be sure to check the description to confirm it is the pure Arabic text rather than an English-only translation.
The book even covers specific protocols, such as how to offer the greeting of peace ( salam ) to the "men of the unseen" ( Rijal al-Ghayb ). Significance in Sufism