I know that Direct3D 9.0 and 10.0 are both not up-to-date APIs, especially Direct3D 10.0 which is subset of 11.0, but I'm wandering whether Frank D. Luna books about older APIs (9.0, 9.0c, 10.0) are still worth reading? For example if there are some techniques or examples, which are not described in the newer books about 11.0 and 12.0, but still relevant?

If I remember correctly, he released a 9.0 book that was using the fixed-function pipeline and then a newer one “Shader Approach”. So, first of all, ignore the fixed-function pipeline book completely.

Altough I have all the books, I don't remember whether there were some good examples in the 9.0c one that weren't in the newer ones. The answer to your question depends on your situation - do you have some of the books? Can you get them for free? If you for example have the 10.0 book for free and you don't have the newer ones, you definitely should read the one you have. In all the books there is a great explanation of the basic principles of 3D graphics (including vector algebra etc.). And also the principles of shadow mapping and other techniques are the same, just the implementation may differ a bit.

If you can get any of the books, then it would be safer to simply get the book about the DX version you are going to use. Otherwise you may find out that for example the math functions mentioned in the book are not available to you (D3DXMath versus DirectXMath etc.), that you cannot load a texture (missing D3DX library), that some API method is called differently than in the book and so on.