The first and the most important source of Seerah is the Qur'aan. It is also the most eloquent and the most authentic source of the seerah. It mentions incidents when the Prophet ﷺ was still a child, and for example, the revelation of Surat Al Feel (The Elephant) referred to the year of the elephant and the time when the Prophet ﷺ was still in his mother's womb.
The Qur'aan therefore refers to incidents throughout his life and refers to the people around the Prophet ﷺ. The people around the Messenger ﷺ like his Companions, his wives, the Mushrikeen (polytheists), the Jews and Christians, etc. The Qur'aan tells you something that no other source can tell you. It tells you the inner thoughts of the people. It tells you how all sorts of people were thiniking and what was actually in their hearts, and the biggest of these example is the hearts of the Munafiqoon (hypocrites) and the Yahood (Jews).
The Books of Hadeeth
All the books of hadeeth are considered an important part of the sources of Seerah. Each and every Hadeeth is either a saying, an action, an event, or a characteristic of the Prophet ﷺ and therefore it becomes a part of the life of the Messenger ﷺ. It is a part of the overall Seerah of RasoolAllah ﷺ and with the Qur'aan this is where most, if not all of the Seerah's information lies.
The Classical Books of Seerah
These are books written by either Companions or their Followers from the early generations who wrote books which are specifically for the purpose of compiling the life of the Prophet ﷺ. The first physical book of Seerah was written before any other book of Hadeeth was compiled.
'Urwah ibn Az-Zubayr (d. 92 AH), the son of Asma bint Abi Bakr (d. 73 AH) (Radi Allahu Anha) and Az-Zubayr ibn Al-Awan (d. 36 AH) (Radi Allahu Anhu), grandson of Abu Bakr (d. 13 AH) (Radi Allahu Anhu), and the younger brother of Abdullah ibn Az-Zubayr (d. 73 AH) (Radi Allahu Anhu), never got to see the Prophet (Sal Allahu Alayhi Wa Sallam). He wrote the first tract on the Seerah that we know of.
Abaan ibn Uthman ibn Affan (d. 105 AH), the son of Uthman (d. 35 AH) (Radi Allahu Anhu) wrote a small book on the Seerah as well.
Also Ibn Shihab Az-Zuhree (d. 124 AH) and Hamman ibn Munnabbih (d. 101 AH), a student of Abu Hurayrah (d. 59 AH), wrote books on the seerah.
Probably the most popular and one of the most comprehensive of books known to is on the Seerah is that of Mohammed Ibn Is'haaq who was born in 85 AH and died in the year of 150 AH. He made it his life long passion to study the seerah. He was one of the first people who put immense concentration and focus on the compilation of the seerah. The scholars of hadeeth call him an alright narrator because he didn't specialize in hadeeth. But they call him the Imaam of Seeratun-Nabuwwah.
There is a clear distinction that we have to make between the Seerah and the Sunnah or Hadeeth. This issue is discussed at another place. The Seerah of Ibn Is'haaq is now known as Seerat Ibn Is'haaq. Ibn Is'haaq grew up around the sons and grandsons of the Sahaabah. He also traveled around the Muslim empire and gathered all the information he can regarding the Seerah. He would put any thing and everything in his book regarding the Prophet ﷺ, whether it was important or unimportant. He would put random poetry that would just mention the Prophet's ﷺ name here and there. This is why this book was so comprehensive.
We see that a lot of times the books of Islaam not known to present day Muslims are found in the museums and libraries of the Europeans because when the Europeans would conquer Muslim lands they would take the most valuable things back to their countries. And at that time there was nothing more valuable than the knowledge these books contained. An example was that, one of Ibn Taymiyya's books was found in the museums of Europe recently which has not been known to the scholars of today. Similarly the manuscript of the Seerah of Ibn Is'haaq was found 20 years ago in the libraries of Germany. Only about a third or fourth of the actual book was found.
Abdul-Malik ibn Hishaam (d. 216 AH) wrote a book on the Seerah and it was call Seerat Ibn Hisham. Ibn Hisham came and summarized the seerah of Ibn Is'haaq. The irrelevant material from Ibn Is'haaq as mentioned previously was discarded by Ibn Hisham in his Seerah. Ibn Hisham mentions this in his introduction to his Seerah and he only kept in his book whatsoever he saw important and necessary. When Ibn Hisham came out with his Seerah it was so good and it became popular very quickly. Everyone who wanted to learn the Seerah had this book and was starting to become more and more common and was being found in each and every household. Because of this, within a hundred years or so, Seerat Ibn Is'haaq was no where to be found. It was almost as if it was removed from the face of the earth, until recently.
Another source of Seerah is the books called Shamaa'il. These are books written exclusively talking about the characteristics of the Prophet ﷺ. In Arabic they are known as Shamaa'il. An example of this is the famous Shamaa'il At-Tirmidhee compiled by the great Muhaddith (Scholar of Hadeeth), Aboo Eesa At Tirmidhee (b. 209 A.H. – d. 279 A.H.). It includes the characteristics and the physical description of the Prophet ﷺ like the way his hair was, the way his eyes were, his walking, his talking, his dressing, etc.
These are books written about the miracles of the Messenger ﷺ. Dalaa'il An-Nabuwwah by Imam Al-Bayhaqee (b. 384 A.H. – d. 458 A.H.) is probably the most famous among them.
Biographies of the Sahaabah
The biographies of the companions serve to make the Seerah more complete and to fill in some holes. They also give some of the vents surrounding it. An example of this is the story of Salman Al Farsee (d. 35 AH) and his search for the truth which will be mentioned later. Also that of Ja'far ibn Abi Taalib (d. 9 AH) and his conversation with Najaasee, the king of Abyssinia.
The Books of History
The books of history serve as another source of seerah. Examples of these books are, the book on history by Ibn Jareer At-Tabaree and Bidaaya Wa An Nihaaya of Ibn Katheer (rahimahullaah).