Here is a list of all related documentation pages:

Preface
Introduction to FLTK
FLTK Basics
Common Widgets and Attributes
Designing a Simple Text Editor
Drawing Things in FLTK
Handling Events
Adding and Extending Widgets
Using OpenGL
Programming with FLUID
Advanced FLTK
Unicode and UTF-8 Support
FLTK Enumerations
GLUT Compatibility
Forms Compatibility
Operating System Issues
Migrating Code from FLTK 1.0 to 1.1
Migrating Code from FLTK 1.1 to 1.3
Developer Information
Software License
Example Source Code
FAQ (Frequently Asked Questions)
Todo List
Deprecated List