The latest revision of the ACPI Specification can be downloaded from the specification and errata download page.
A basic ACPI overview presentation in is available here:
Title of Presentation:
Select the icon of the type of file to view, either as an Adobe Acrobat file (PDF: Acrobat Viewer required) or a Microsoft PowerPoint file (PPT: Internet Explorer 4.0+ required).
Company: ACPI Overview