If anyone in the English-speaking world (particularly in the USA) would like to learn Mongolian, the
American Center of Mongolian Studies (part of the Center for East Asian Studies at University of Pennsylvania) might be of use. It runs summer classes for Mongolian in Mongolia (I think that you need to be a student at UPenn but the classes aren't cheap) and also has a
bookstore where you can order fairly rare textbooks and reference material for Mongolian. The site also used to host an extensive series of free online lessons in Mongolian (at least 20 sections if I remember correctly) covering material up to what the ACMS deemed intermediate (A2? B1?) but they've been gone for several years already. A shame.
Of the rarer books, I do have the first volume of the 3-part series
Сайн Байна уу? and its accompanying audio. All of the audio is available at Indiana University's
CELT portal but you need to be a registered student there. I remember a time when a lot more audio on the archive was available to the public even if that audio was part of a book that was (is) still under copyright. Anyway, the book itself is meant for an English-speaking learner attending Mongolian classes although the appendices have transcripts and translations of the dialogues and the answer key covers about half of the exercises in each chapter. It could be used by a fairly motivated independent learner although I think that it should be supplemented with another course, or even better, used with the help of a Mongolian tutor/friend.
On the "Colloquial" series in general, I think that Speakeasy is unduly harsh by considering the series as meant to provide the user with CEFR A0 competency of the target language. That's too damned low and, with all due respect, wrong. A lot of the material in the second half of a regular "Colloquial" language course would be covered by students at A2 if not B1; it'd be beyond what someone at A1 (let alone A0) would be expected to know. After having used several of the books myself (and also having "Colloquial Mongolian" on the shelf), I've gathered that most are similar to most volumes of the "Complete" series from "Teach Yourself" which are honestly assessed as taking the learner up to CEFR A2
(in other words, most users can move on to struggle with a textbook advertised for users starting at B1(.1)) in their German adaptations/translations published by
Cornelsen. The "Teach Yourself" books are, however, dishonestly assessed as taking you up to B2 in their English "Complete" editions
(that's just false advertising as there's no way that anyone having just finished only a Teach Yourself language book will then have a fighting chance using a textbook targeted at users with competency at B2 let alone at C1).There are a few Mongolian resources in Russian (look up монгольский язык, учебник монгольского языка and similar) but like what we can get in English, they vary from stuff issued during the Cold War (they're published in cities followed by CCCP (USSR)) to stuff issued this century although they're all meant for classroom instruction. The variety of material is limited.