Like others, I would suggest K&R. Perhaps after that, Lourdon's "Mastering algorithms with C". I've found that structuring any program around well understood datastructures and keeping it KISS after that takes you a very long way. Also applies to C. Like others have commented, stylistically the code in the linked book is perhaps not the best place to learn the best conventions for C. But! Writing an interpreter gives a huge kick and reading other peoples code is always nice.
For real world projects, it's not a bad idea to implement your own memory manager (even if it's only a wrapper for malloc and free). For real world interpreters Lua (www.lua.org) is written in C, is quite widely used and is not too large - if interpreters are your interest. If you are into lisps Structure and Interpretation of Computer Programs http://mitpress.mit.edu/sicp/full-text/book/book.html contains a the instructions for writing a lisp interpreter in any language.
I disagree with recommendation of Lua. Lua is a production-grade interpreter, and a production-grade interpreter is usually not a good educational material for those "interested in interpreters".
In particular, Lua uses no intermediate representation (there is no Lua AST!), which makes its compiler portion a quite confusing read. Its interpreter portion is no less confusing, being a register VM with a register window scheme and a non-recursive eval using goto and a separate call stack. Do not be fooled by "not too large" aspect. Lua is worth studying, but it is definitely not a beginner material.
For real world projects, it's not a bad idea to implement your own memory manager (even if it's only a wrapper for malloc and free). For real world interpreters Lua (www.lua.org) is written in C, is quite widely used and is not too large - if interpreters are your interest. If you are into lisps Structure and Interpretation of Computer Programs http://mitpress.mit.edu/sicp/full-text/book/book.html contains a the instructions for writing a lisp interpreter in any language.