This was one of the Teach Yourself series of the mid-1960s, running from Teach Yourself Afrikaans to Teach Yourself Welsh. I'm not really sure who it is aimed at; the first section decribes the motivations one might have for learning a language, and learning it well, but in such general terms that it's difficult to imagine anyone finding it helpful in their own specific situation.
The second section is rather more interesting: brief surveys of eleven major world languages (Arabic, Chinese. French, German, "Hindustani" = Hindi + Urdu, Italian, Japanese, "Malay" = Indonesian, Portuguese, Russian, and Spanish) explaining the bits that an English speaker might find difficult or easy. It does have some interesting judgements (German "suffers from the serious disadvantage of having a great deal of useless luggage of language - case endings"; Portuguese "has progressed further than the other languages of the Peninsula in losing certain consonants in certain positions") but as long as one bears in mind that the whole book is written in a style which would have surely seemed old-fashioned in 1965, it is enlightening - I had not realised, for instance, that Arabic has essentially only two tenses, or that Chinese adverbs are formed by doubling the adjective and adding a particle.
The final section gives basic word lists for Chinese, Italian and Spanish (odd choices) and then makes a very good point in the last couple of chapters: that basically there are only two stages of language learning, before "the penny drops" and after. While it is important not to kid yourself that you have reached that point of departure before you are really there, it is also important to plan ahead for the moment when you are ready to pick up newspapers and books and listen to the radio. Indeed (though Glendening doesn't say this) it's probably by keeping that goal in mind from the very beginning that you can best motivate yourself.
Right, where's that Penguin Russian Course gone to...