Modern Macedonian is a curious little language: it is a literary standard created in the 20th century for a group of South Slavic dialects spoken by people whom Bulgarian officials call "Macedonian Bulgarians", and Serb politician used to call "Southern Serbs". The actual language is a lot like Bulgarian, but there are a lot of differences in spelling between the standard written forms of the two languages (e.g., Macedonian often has a v or an f, or nothing at all where Bulgarian has an h), as well as some specifically Macedonian words, so that trying to look up Macedonian words in a Bulgarian dictionary would not be practical for most people. It is also nice to have a concise "cheat sheet" for Macedonian grammar, explaining the conjugation of verbs, the forms of pronouns (which, too, are often different from Bulgarian), etc.
There are good Macedonian dictionaries on the market, such as the weighty English-Macedonian, Macedonian-English Standard Dictionary (ISBN 9989809356), which is more complete than Usikova's volume, but also is a lot heavier; there is also a very good online dictionary. There are textbooks and grammars targeted to the English-speaking readers as well, such as a good academic grammar by Olga Mišeska Tomić (ISBN 089357385X). The fact is though, if you already grasp Russian grammar and have a good Russian vocabulary, you don't need to read the 500 pages of Tomić (or a similarly sized book on Macedonian grammar, in Russian, authored by Usikova herself). The 40-page grammar reference in the back of Usikova's dictionary, with handy conjugation tables etc, together with the dictionary itself, would let you read pretty much anything published in the Republic of Macedonia fairly easily. In my experience using this dictionary with a couple of Macedonian books or articles, I'd run into a stumbling block maybe once in 5-10 pages, and then a reference to a bigger (online) dictionary (if I can't guess the meaning of a word) or to Tomić book (if the grammar is particularly tricky) would often be helpful. And a major advantage of Usikova's book is that, while certainly not pocket-sized, it is still small enough to be fairly convenient for travel use.
Obviously, the book is written primarily for native Russian speakers, but anyone who's achieved a decent reading proficiency in Russian and now wants to "diversify" to another Slavic language can make a good use of it as well.
From a user's point of view, one certainly can slightly expand the vocabulary contained in this dictionary. Among possible candidates for additions I can list, for example, some words frequently used by Krste Misirkov in his famous "Za Makedonskite Raboti" (pretty much the first book ever written in modern Macedonian), such as arno ('good'), as well as some recent Serbian loanwords (?), such as točak (originally 'wheel', but seems to usually mean 'bicycle' in Macedonian). But, overall, the dictionary is quite adequate.