A paperback copy of The C Programming Language costs $52.34 on Amazon
http://www.amazon.com/Programming-Language-Brian-W-Kernighan/dp/0131103628/ref=sr_1_1?ie=UTF8&qid=1442792912&sr=8-1&keywords=the+c+programming+language
Amazon also sells their "Fire" tablet for $49.99, which runs a Linux-based operating system (so not strictly Unix, but close enough) http://www.amazon.com/dp/B00TSUGXKE/ref=ods_gw_d_h1_tab_frd_LG9_TagD?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=desktop-hero-kindle-A&pf_rd_r=0XVBJV6JJT07JPK2K4M3&pf_rd_t=36701&pf_rd_p=2211604862&pf_rd_i=desktop