Assimil is a French company, founded by Alphonse Chérel in 1929. It creates and publishes foreign language courses, which began with their first book Anglais Sans Peine ("English Without Toil"). Since then, the company has expanded into numerous other languages and continues to publish today.

Their method for teaching foreign languages is through the listening of records or tapes and the reading of a book with the text that you are listening to, one side native language, one side foreign language. This method is focused on learning whole sentences, for an organic learning of the grammar. It begins with a long passive phase of only reading and listening, and eventually adds active exercises. Most books contain around 100 lessons, with the active phase starting on Lesson 50. By the end of the "With Ease" Series you'd have a working vocabulary of around 2000-3000 words, depending on the language youre studying.
Here is a great review by Prof. Alexander Argüelles.
Unfortunately, there languages available in English is relatively small compared to the ones available to French, German and other Europian speakers. Currently those available to English students are Arabic, Chinese, Dutch, French, German, Hungarian, Italian, Japanese, Russian and Spanish. If you want to learn other languages such as Swedish, Hindi or Turkish for example you'd need to learn it via their French or German editions.

The older courses from the 1950s ("Without Toil") seem to have more material in them. Making the older versions in just as much a demand as the modern ones.
My rating:
Unfortunately, I am no longer able to sell Assimil products on my website. But you may find them cheaper on eBay or Amazon.

Alternatively, send me an email and I can help you find them cheaper.