I have been collecting these old Pelican books for a few years now.
Even though they are older books and some content is outdated, they are still informative and good introductions to various subjects. Additionally, several of these are still in print.
My first books were Introducing Shakespeare and A Short History of English Literature. These were soon followed by one or two of the history books.
Nowadays I just buy any volumes that I happen to find, no matter the subject.
My most recent acquisition was the Early Victorian Novelists which I am currently reading.