Well...
If you want to get deeper into Tolkien's works than that of The Hobbit and The Lord of The Rings, you need to read these:
The Hobbit
TLoTR
The Silmarillion
Children of Hurin --- This is not all that necessary, but I recommend reading it. It's not big (about 300 pages).
The Unfinished Tales
The Letters of JRRT.
That's my recommended order. I would also throw in Tales From the Perilous Realm before or after The Unfinished Tales.
The Hobbit, TLoTR, and The Silmarillion will put on you solid ground when it comes to Arda (Tolkien's universe). The tale of Hurin is included in The Silmarillion, but it is a shorter version of the book. Unfinished Tales consists of a ton of background history and tales from both Middle Earth and Numenor. The Letters of JRRT is important to understand why Tolkien explains things as he did, and sometimes he will elaborate on certain characters or events and so forth.
The History of Middle Earth is also important, but it requires a ton of motivation to go through. And yes, there are 12 books, so it will take time to go through it all.
At a normal reading pace, you should be able to get through the above list (excluding The History of ME) in three months or so, depending on how much you read. But trust me, it takes motivation.
The History of ME, The Silmarillion, The Unfinished Tales and Children of Hurin (or rather, Narn i Chîn Húrin) were all posthumously published, so it's evident that they are not as easy to read as The Lord of The Rings.
And by the way, when you have read them all, you should go back to The Hobbit and TLoTR; you will understand them both a lot better.