How to construct complex queries

Description:
Is there more information available on how to construct complex queries?
For example when I have 3 entities (A,B,C) with associations: [A] 1---->n [B] m<----1 [C]
How do I efficiently query all [A] entities which have a relation with [C]?

Answer:
Please review the How to Compose Complex Criteria help topic. It describes how to create complex queries.
As for your particular task, you can use the following code:

C#
 public class Task : XPObject {  
  [Association("TaskAssignmentsTask", typeof(TaskAssignment))]  
  public XPCollection TaskAssignments {  
   get{ return GetCollection("TaskAssignments"); }  
  }  
 }  
 public class TaskAssignment : XPObject {  
  [Association("TaskAssignmentsTask")]  
  public Task Task;  

  [Association("TaskAssignmentsPerson")]  
  public Person Person;  
 }  
 public class Person : XPObject {  
  [Association("TaskAssignmentsPerson", typeof(TaskAssignment))]  
  public XPCollection TaskAssignments {  
   get { return GetCollection("TaskAssignments"); }  
  }  
 }  

new XPCollection(typeof(Task),  
   new ContainsOperator("TaskAssignments",  
    new GroupOperator(  
     new BinaryOperator("Person", myPerson),  
     new BinaryOperator(  
      new OperandProperty("Task"),  
      new OperandProperty("^.Oid"),  
      BinaryOperatorType.Equal  
     )  
    )  
   )  
  )  

See also:
What criteria should I use to get objects for a collection property which is empty?
How to set query criteria for a collection which is a collection property of a persistent object
How to filter a collection by an associated objects' properties