What does Jethro send to Shad that are" like gifts to a lonely boy" in Across Five Aprils?

It is actually Jethro who is the "lonely boy," and Shad who sends things that are "like gifts." These things are letters, which Shad writes from Washington while he is recuperating from injuries sustained in the war.

Jethro's brothers have all gone off to fight in the war, his brother Bill for the Confederacy, and the others for the Union. Shadrach also has been fighting for the North, and is wounded at Gettysburg. Jethro's sister Jenny is in love with Shadrach, but her father does not allow them to marry before Shad goes away to war, because he thinks Jenny is too young. When Shadrach is gravely wounded, Mr. Creighton relents and allows Jenny to go to him, and to become his bride. With all his brothers and his sister gone, Jethro is left with his parents on the farm alone, and although his days are filled with hard work and studying the books Shad has left him, there is not doubt that he is lonely.

When Shadrach is "slowly struggling back to health" in Washington wth Jenny by his side, he writes letters to Jethro. These letters are

"like gifts to the lonely boy; he read(s) them over again and again, and then place(s) them carefully in (a) big envelope..."

Shad writes about the war and politics in Washington, and his own reflections about what is happening. In particular, he writes about President Lincoln, whom both he and Jethro love and admire greatly (Chapter 11).