The "Great Siege", is the Siege of Malta in 1565, as the Turkish Ottoman Empire tried to expand further into the Mediterranean and up to Italy. The Ottomans had already conquered most of Eastern Europe.
The book, by a British historian named Ernle Bradford, is great! But unfortunately extremely difficult to get. It's not stocked on Amazon and second hand copies are difficult to get. I was lucky and got mine second hand off Amazon's website for 20 bucks plus shipping, back in April. It's a book I'd always had a wish to read, since seeing a review years ago.
The historical background to the siege, and an abbreviated discussion can be found here:Siege of Malta (1565) - Wikipedia, the free encyclopedia.
The book uses all the contemporary accounts and puts them into a flowing narrative, that is really quite riveting. The main characters are the Grand Master of the Order of the Knights Hospitaller of St. John, (a fighting religous order who also maintained hospitals! Go figure.), Jean de Valette, the Turkish leader , Mustafa Pasha, and his Tripoli ally Turgot Reis.
The Turkish invaders really should have won the day as they had vastly more men. They were stymied by their own infighting, some bad tactical decisions (especially opening the siege by trying to capture the Fort of St. Elmo's), and by the heroic defense of the Christian defenders who travelled to Malta, and the Maltese fighters. The violence level is appalling. It was a bad idea to be captured, by either side!
It's a great, highly readable story, if you can get the book. I hope it gets re-issued soon.