<p>I am a fresh graduate student in 3D metal printing. My undergraduate major is mechanical engineering. Later research will focus on the process of metal 3D printing. I hope that you can recommend some excellent 3D metal printing books for learning.</p>

This is a free ebook that I have perused briefly which it looks interesting, and it is free (did I say that already?)
3D Printing of metals
Manoj Gupta
ISBN 978-3-03842-591-5 (Pbk);
ISBN 978-3-03842-592-2 (PDF)
Three other books that
might
be of interest are:
3D Printing with Metals for Design Engineers, Explained
Ann R. Thryft
Downloadable free ebook, but some sort of sign up is required
Additive Manufacturing of Metals: The Technology, Materials, Design and Production
,
Yang, L., Hsu, K., Baughman, B., Godfrey, D., Medina, F., Menon, M., Wiener, S.
ISBN 978-3-319-55128-9
Additive Manufacturing of Metals: From Fundamental Technology to Rocket Nozzles, Medical Implants, and Custom Jewelry (Springer Series in Materials Science)
Although, as the title contains a (rather obvious) mis-spelling, it does not bode well for the rest of the book.
John O. Milewski
ISBN-13: 978-3319582047
ISBN-10: 3319582046