The Lorax is one of Dr. Seuss's more serious books which a crystal clear message: save the planet. The Once-ler enbodies blind capitalism only looking to become bigger and bigger, not caring about the fluffly trees, nor the animals, nor the smog. Furthermore, he ignores the Lorax who warns him of the effect of his factory. The Lorax then is, in a sense, the charitable organisations who look after the well-being of nature, animals and Earth in general.
Apart from the damage he caused, and the fact that he ignores the warnings of the Lorax time after time, valuing money over nature and health, there is another message - more veiled perhaps - one of love and happiness. When the Once-ler recognised the potential of his 'Thneeds, which everyone needs', he called all his family to help out in this new family business. However, when the felled the very last colourful Tartuffa Tree and there was no more fluff to make Thneeds, they packed up their stuff and left.
Alone and unhappy, worrying about how dumb he was to ignore the Lorax's warnings, with all the money he supposedly earned, he spends his days in the ruins of his former factory. Being a cross between Boo Radley and a local wise druid, he waited for this one boy to come and listen to his story to give him the very last Tartuffa seed - to save the world. No pressure.
I decided to read this after seeing the film, but was surprised to find that they had added an entire new storyline of the pseudo-utopia where the boy lives. Compared to the film, the book is very one dimensional. Nonetheless, the lovely language and ridiculous rhymes of Dr. Seuss are as delightful as ever. I would recommend you read the book before watching the film because you don't want to regard the book as a kind of anti-climax.