A quick and easy way to join or merge multiple PDFs together on Linux (Kubuntu 14.10):
pdfunite toc.pdf chapter1.pdf chapter2.pdf book.pdf
This doesn't resample pages or any other nonsense like that either :) pdfunite is provided by the poppler-utils package which should already be installed.
If you want to do the opposite, i.e. split a unified PDF file into multiple ones, poppler-utils also provides the aptly named pdfseparate.
Hope this helps :)