Depending on how involved the language you are interpreting is, you might get by having only read chapter 6 of The AWK Programming Language (linked in the article), which covers "Little Languages", including what it terms an assembler and interpreter.
If you are interested in more depth, either Crafting Interpreters (mentioned in the article) or Writing an Interpreter in Go looks promising. I've read more of Crafting Interpreters and really enjoy it, though it isn't yet finished. One of the aspects I really enjoy is that the language is implemented and re-implemented in different languages to gradually introduce lower level concepts.
Finally, this one may be a little more "out there" than what you are looking for, but if you are interested in designing a language more than the plumbing of an interpreter Beautiful Racket is really good.
caveat: not an expert
It's available online. Thanks for the tip.