The Hobbit is a so-called Prequel to the Lord of the Rings, which tells the story of Bilbo the Hobbit discovering the Lord of the Rings and his adventures.
The Lord of the Rings itself is divided into three parts (originally Quan Yi's book), namely the first The Lord of the Rings, the second towers, and the third Return of the King.
Silmarillion, a myth and legend of Middle-earth, tells the history of the Lord of the Rings War for more than 10,000 years from the beginning of heaven and earth. The composition of the novel is low, and it is biased towards the nature of manuscript compilation. But some stories are wonderful and vivid. It should be pointed out that it has nothing to do with the story of The Lord of the Rings. The so-called "the origin of the Lord of the Rings" is a gimmick made by publishers, so don't believe it.
The above is the main line of Tolkien's middle-earth story (that is, the five novels commonly known as "The Complete Works of the Lord of the Rings"). The reading order is the Hobbit first, and then the The Lord of the Rings Trilogy. As for Poké mon Diamond, you can read it after you have a preliminary understanding of Middle-earth, because its content is difficult to understand.
In addition, the story content of the newly published Children of Lin Hu comes from Poké mon Diamonds, which is an expanded version of a story in Poké mon Diamonds. But it does not belong to the category of "The Complete Works of the Lord of the Rings".