The journals that James Cook during his three Pacific Voyages were edited by others, and published as the official accounts of these voyages..
Many other people have written about James Cook, his ships, voyages, his life, the people who went with him, those he met, and the places he saw.  Reviews of over 200 of these books can be found on this website.
If you would like some guidance on which books to read, then take a look at the ones that author Tony Horwitz found of great use.
Finally, a bibliography is provided of almost all of the books published over the last 55 years.