Tree ring cut of a red cedar that has been properly torched, brushed, sealed, and is ready for ink.

There are many books that are very helpful for improving your printing. 

The first I'd recommend is the Nature Printing Society guidebook, Printing From Nature. It has chapters from various members and is the single greatest resource on the subject.  

For botanical printing, I'd recommend John Doughty and Sonja Larsen's book Creating Art from Nature

NPS member Sue Fierston just came out with a new book, Into the Woods: Families Making Art in Nature. It has a lot of great ideas and techniques from a seasoned artist and instructor. 

For printing stumps and woodcuts, Bryan Nash Gill's book, Woodcut is an helpful and inspiring resource. 

More and more people are using Youtube as a source. I've begun my own channel and will be updating it with information about printing. I have an old video about making tampos that isn't the best filming, but the info is solid.