There are no required textbooks for CS 146. Students learning C are strongly advised to buy at least one of the two C textbooks listed below.
The most user-friendly option is the CS 136 textbook, "C Programming: A Modern Approach" (CP:AMA), second edition, by King, which is available at the UW Bookstore. Your instructor chose this book for CS 136 after reading many textbooks; it was the best of the lot.
Students with considerable prior experience in computing may be able to use "The C Programming Language", second edition, by Kernighan and Ritchie, also available at the UW Bookstore. This classic book, however, is considerably more terse, and does not include details of the C99 standard. C99 features are used in a minor fashion in the course materials, and they are pointed out when used.