There are many fantastic books on Unix. But one of my favorites is Unix Text Processing by Dougherty and O'Reilly (of the O'Reilly Books fame) which came out in 1987 under the banner of Hayden Books. So much of Unix is text processing and this book deals with the plethora of text manipulation tools with unmatched clarity. Its short but focussed chapters on the vi editor and sed are worth a read even today.

Another reason why this book remains popular is its exhaustive coverage of troff and the ms, mm macro sets. No text since has tackled this fantastic tool which is a pity. I wrote the first edition of my text on SQL using troff and the ms macros and the output was top notch. It takes some time to get used to though, if you come from modern sensibilities.