Greetings,
I have started learning struts through this online PDF format file known as Jakarta Struts-Live.. You can get the PDF and Src files via this website link,
Jakarta Struts-Live If you are looking for a book based on Struts, you can try "The Complete Reference, Struts" which is authored by James Holmes and edited by Herbert Schildt.
I find this two sources quite good for starters of Struts... but, do not depend much on either one of them. My recommendation, get both!