Some professional organizations (like the American Physical Society or SPIE) maintain digital libraries where you can search for articles published in their journals.
Databases that aren't tied to a specific publisher let you search for articles and conference proceedings from a wide range of sources at one time.
Books are helpful when you want an introduction to or background information on a specific topic. You can search for print and electronic books using the One Search tool at the top of this page.
Here are some areas you can browse books on selected topics inside the library:
QC174: Quantum Mechanics
QC176: Solid State Physics
QC717.6-718.8: Plasma Physics
QC793-793.5: Particle Physics