Textbook Recommendations Covering The Adaptive Immune System
There are two books that I see as "standards" in the field:


Cellular and Molecular Immunology, 8th Ed. by Abbas et al.
Immunobiology previously by Charlie Janeway, who unfortunately has passed, so it is now authored by Kenneth Murphy. Many people just refer to it as "Janeway".


These texts can be used both at an undergraduate and graduate level (I used them in both), and are updated every few years. The latest edition of Immunobiology is from 2011, while CMI is from 2014, so you may want to start with that one first. 

An earlier version of Immunobiology is also available for free on NCBI's Bookshelf, but unfortunately you can't read directly through it, you'll need to search for what you're interested in.