There are many sources on the life of Sri Guru Gobind Singh Ji such as
Gurbilas ( 10)
Suraj Parkaash
Sri Guru Sobha
Sri Guru Granth Parkaash
Panth Parksaah
Bansvalinama
Sau Sakhia
Guru Kian Sakhian
These are the oldest writings of Sri Guru Ji ; however, when reading them we have to do so with a vigilant eye as some of the material is strecthced and not in accordance to Gurmat. Out all the above readings I reccommend Sri Guru Sobha as its much more cleaner then the other material I have mentioned plus its the most detailed of Sri Guru Jis last days and most detailed on the battles.People say that Gurbilas ( 10) is the best biography of Sri Guru Ji as the life events are in chronological order. Personally, I see the texts has some good information but has information which is totally contrary to Gurmat such as Sri Guru Ji worshipping a devi and 5 goats beings slaughtered instead of Panj Pyaarey.. There are some other books about Sri Guru Jis life but I cant think of their names at this time. When I do I will be sure to notify you.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥