m_jawza put in a bit of time to do a useful review of this book, which he has sadly since deleted. He'd used the book to try to learn to read and write Persian as a US-born native speaker.
He starts: "Take it from a fluent speaker [...] Lambton's book is probably NOT what you should be looking for if you want to start learning the language." And if you want to speak Persian, I agree!
Lambton's Persian Grammar is an excellent book on literary Persian. It is an academic work of the nineteen-fifties. It describes sounds well to anyone who can follow the standard linguistic descriptions (yes, it could be more accessible). It is hardly surprising that forms like 'bachegan' - traditionally the plural of 'bache', child - might confuse a speaker who doesn't know literary Persian since they no longer appear in the spoken language. But the book is teaching literary Persian, and if plurals in '-gan' aren't in, you're going to be very confused when you come to any pre-modern or very formal writing. And Lambton does indeed say that plurals in '-an' can always be replaced by plurals in '-ha' and almost always are in speech.
I think, though, that m_jawza's biggest problem is going to be that the book is fifty years old. Persian has changed a bit in the last fifty years! But by the time he finishes, he'll be able to pick up a ninteenth-century book and have a chance of understanding it (with a copy of Steingass - or the Loghat-name - for all those Arabic and out-moded words). I'm not so sure about writing, though. I wasn't very impressed with the handling of syntax. I think for that, he'd do better to use Wheeler Thackston's 'an Introduction to Persian'.
By the way, I certainly agree that she packs a lot in one lesson. I too found the definite/indefinite description unclear. Lambton's book is a very traditional teaching grammar that can double as a basic reference grammar - indeed, it is the standard grammar of written Persian. I really preferred Wheeler Thackston's approach. But I use Lambton as a reference, and it is very good for that.
If you are wanting to use Lambton for self-tuition, two points, one good and one bad. She does have a key to the exercises, which Thackston doesn't. But she doesn't have a glossary - that, expanded, is published as a different book, Persian Vocabulary. Both volumes, by the way, are available at a considerably lower cost in paperback editions published by Chand of New Delhi that are actually better bound than the current Cambridge editions.
BTW, m_jazwa, please do come back to your review when you've used Lambton a bit more - I for one would like to know how you get on!