If I have a decent Hebrew knowledge, but not great, what books (i.e. dictionaries, lexicons, etc.) should I have at hand when I'm learning a sefer, such as the Mishneh Torah?
In addition to Shalom's recommendations for dictionaries:
The Practical Talmud Dictionary, by R' Yitzhak Frank. It's perfect for beginners because it presents phrases as they're actually found in the Talmud and explains what they really mean, in context. (As in Jastrow, most of the words are Aramaic rather than Hebrew, but they're relevant to the study of Hebrew Rabbinic texts, since they tend to quote Talmudic passages or phrases frequently, and use a Hebrew that borrows many words from Talmudic Aramaic.)
A book of acronyms, usually called something like "Otzar Rashei Teivot." (Here's an example, but I've seen other versions, and they probably all do the trick pretty well.) Rabbinic books from every age contain tons of acronyms and contractions, and they can be one of the biggest pitfalls for someone who tries to understand the text without extensive experience.