You should understand how BOINC works (for both participants and projects) before getting into the source code.

Read about:

• Compiling BOINC software
• BOINC coding style

Various implementation notes:

Core client

• File structure
• FSM structure
• Data structures
• Main loop logic
• Client scheduling policies
• Debugging
• Host measurements
• Host identification
• Core client/application interaction (basic)
• Core client/application interaction (graphics)

Server programs

• The BOINC database
• Work distribution policy
• Backend state transitions
• The logic of backend programs
• Debugging server components

Protocols

• Protocol overview
• The scheduling server protocol
• Scheduling server timing and retry policies
• Data server protocol
• Persistent file transfers

Miscellaneous

• Python framework
• Preferences
• Trickle messages
• How to see what has changed between two versions of an executable.
• Account management systems