Can you recommend professional 3D metal printing books?
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