His book, Muruj Al-Dhahab, known in English as The Meadows of Gold and Mines of Gems, is a historical account of the beginning of the world up through the Abbasid Caliphate in Baghdad. You can read more about the book and its translations at this wikipedia page I created: http://en.wikipedia.org/wiki/The_Meadows_of_Gold
What is interesting about the book to me and Nights scholars is that it is one of the first books to mention the Nights at all, and dates the Nights to the 10th century, if not suggestively before then. There are only four known mentions of the Nights apart from any versions of the collection pre-1400s-ish, Masudi's is one of them.
The mention, which is very brief, contains information that the Nights is an Arabic translation from a Persian story collection known as Hazar Afsaneh, that it is known in Arabic as The Thousand Nights and a Night, and that it is the story of a King, his Vizier, the daughter of the Vizier who is named "Shirazad", and her handmaiden/slave, a girl named "Dinazad".
All of which are important details for Nights scholars in many different ways.
Here is the rendering in Arabic from the French and Arabic version of Muruj (important to note because it is not an original manuscript, but was (and is in many ways) the primary source for the Masudi book in Europe and other non-Arabic speaking countries) called Prairies d'or, Arabic edition and French translation of Muruj al-dhahab by Barbier de Meynard and Pavet de Courteille, Paris 1861-77. The following is from volume 4, pages 89-90 (available online through archive.org linked at the wikipedia page above) -
وان سبيلها سبيل الكتب المنقولة الينا والمترجمة لنا من الفارسية والهندية والرومية سبيل تأليفها ما ذكرنا مثل كتاب هزار افسانه وتفسير ذلك من الفارسية الى العربية ألف خرافة والخرافة بالفارسية يقال لها افسانه والناس يسمون هذا الكتاب ألف ليلة وليلة وهو خبر الملك والوزير وابنته وجاريتها وهما شيرازاد ودينازاد ومثل كتاب فرزه وسيماس