PDF Stitcher is a tool that will stitch together PDF files. You can either give it a web page containing a list of PDF files or drag-and-drop files onto the app and it will stitch them together into a single PDF file.
It's great for compiling free textbooks available online as a list of chapters, such as Foundations of Computer Science for example, or multiple PDF files printed from other sources.
This is also hosted as a service available at http://www.pdfstitcher.com.
PDF Stitcher is written in Ruby, uses Bundler to handle gems and leverages Ghostscript to stitch PDF files together.