Contains information on the theory of relational databases, popular RDBMSs, concurrency control etc. The site has a collection of PPT/PDF slides that are useful for repetition of the main concepts.
The Information Retrieval Methods PDF by Christopher D. Manning, Prabhakar Raghavan and Hinrich Schütze is not only gratis, but a good resource on all kinds of Information Retrieval topics: the boolean model, the vector space model, tf-idf weighting, language models etc.
The Linux Administration Handbook (second edition) is $42.89 on Amazon. From the
/proc pseudo filesystem to information on user accounts and GRUB, this book is one of the most comprehensive Linux sysadmin resources I own.