I understand you're looking for free Kindle books by Roald Dahl, but I need to provide an important clarification first: (he died in 1990, so copyright lasts until at least 2060 in most countries). Legally free full-length Kindle editions of his popular books like Matilda , Charlie and the Chocolate Factory , The BFG , or James and the Giant Peach generally do not exist unless offered temporarily by Amazon or a library.
Would you like a quick guide on setting up Libby with Kindle?