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Very roughly speaking, the size of any non-fiction book is determined by two competing factors: the publisher wants to print as little as possible to reduce costs (not just printing, but writing, editing, layout etc) and sell it at the highest price possible, while the reader wants to buy as big a book as possible at the lowest price possible. And the marketplace works out an equilibrium somewhere in the middle.
For guidebooks (and phrasebooks), the amount of content necessary is determined by the size of the place and its popularity with tourists. Mongolia is small (populationwise), rather obscure, and there isn't a whole lot of competition, so books will tend towards the skinnier end of the spectrum.
Source: I used to work for several guidebook publishers.
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Have you tried "Modern Mongolian: A Course Book" by Dr John Gaunt? It's very straightforward, pronunciation of every letter is compared to a similar sound of English words and the grammar explained clearly, not in complicated ways as some other books do. Sometimes they sell copies with audio tapes included. (Sorry, I repeated my answer here). I found Mongolian Phrase books are usually not very satisfactory. Probably there are some ways to have language lessons via Skype or something?