I'm not sure what you mean by the decline of the Ottoman Empire, as there are whole load of perspectives on when/if there a decline actually occurred. While traditionally historians referred to the period after the reign of Suleiman the Magnificent as a period of decline, I've also seen historians who refer to the 18th and 19th Centuries as periods of decline. Today most historians of the Ottoman Empire reject the decline thesis, instead positing that the Ottoman Empire underwent periods of transformation and change, adapting to their changing circumstance rather than simply experience a stagnation or decline.

As for your request for books, I haven't read much on the Tanzimat period so I can't help you there, but here are some recommendations anyways:

1. The Ottoman Empire, 1700-1922 by Donald Quataert: This is a nice general overview of the period in question, it covers mostly economic and social history, but most importantly it covers the notion of a decline in the Ottoman Empire and questions it

2. Ottoman Wars, 1700-1870: An Empire Besieged by Virginia Aksan : A nice look at the Ottoman military record throughout the period, though mainly looking at warfare in the 18th century it also covers a lot of the political developments throughout the late 18th and 19th Century, such as the Tanzimat reforms and their effects on the military organization of the Ottoman Empire.

3. The Ottoman Endgame: War, Revolution, and the Making of the Modern Middle East, 1908-1923 by Sean McMeekins : This book is a pretty good look at the late Ottoman Empire starting with the reign of Abdul Hamid II and looking at the lead up to the first world war from the Ottoman perspective

4. The Berlin-Baghdad Express: The Ottoman Empire and Germany's Bid for World Power by Sean McMeekins : Also by Sean McMeekins, this event focuses on German-Ottoman relations leading up to World War I, and the construction of the Berlin-Baghdad railway.

5. Arming the Sultan: German Arms Trade and Diplomacy in the Ottoman Empire Before World War I by Naci Yorulmaz : Another book focusing on German-Ottoman relations, this time through the lens of German Economic Imperialism and domination of the Ottoman Industry(In particular the arms industry) during the reign of Abdul Hamid II and into the Young Turk era

6. The Ottoman Scramble for Africa: Empire and Diplomacy in the Sahara and the Hijaz by Mostafa Minawi: A good rebuttal to the idea that the Ottomans retained a passive role in the age of Imperialism, it's a good look at Ottoman foreign policy towards areas they viewed as their colonies, and also attempts at the acquisition of new colonies.

7. Death and exile: the ethnic cleansing of Ottoman Muslims, 1821-1922 by Justin Mccarthy: A look at the expulsion and migration of Ottoman Muslims as the territory of the Ottoman Empire began to recede. These expulsions, played a lot into the mindset of Ottoman decision makers throughout the 19th and early 20th century, and played a huge role in the formation of the Turkish national identity so it's worth looking at. The author has his biases, but even still it remains the best book on the subject

You may also want to look at Partners of the Empire: The Crisis of the Ottoman Order in the Age of Revolutions by Ali Yaycioglu and A Brief History of the Late Ottoman Empire by M. Şükrü Hanioğlu as they've been recommended to me a few times, but I haven't read them as of yet.