It depends on what you'd like to know. Above answers suggest Davidson and Blackadar for C*-algebras, and Takesaki for some more detailed theory of von Neumann algebras. These are excellent suggestions.
I'd like to add that if you want to learn more about finite von Neumann algebras/factors, you may find it helpful to read chapters 6-9 of Kadison and Ringrose, and then have a look at Finite von Neumann algebras and MASAs by Sinclair and Smith. Then you perhaps should try to read some of Sorin Popa's recent papers and begin backfilling knowledge around these.
