There are 2 books by Thomas M O'Brien on BookOrb. A strong starting point is To know a tree (A Little owl book) (1963). The bibliography below is in publication order, with series grouped in reading order.
Standalone books
Frequently asked questions
How many books has Thomas M O'Brien written?
BookOrb lists 2 books by Thomas M O'Brien.
Where should I start with Thomas M O'Brien?
A good starting point is To know a tree (A Little owl book) (1963).
What is the Thomas M O'Brien books in order list?
In publication order: 1. To know a tree (A Little owl book) (1963), 2. General Orders of the War Department (2015).