Sometimes it is difficult to get SAP ebook for free due to different factors as copyright, there are some sites where you can get old editions for free however you have a high risk to catch a virus. In facebook there a people who sell ebooks with a low cost.
SAP Press books have an outstanding content about SAP new technologies nevertheless are so expensive and sometimes its validity is for a short time, you can find out more on www.sap-press.com.
SAP Manuals are exclusive for students who take SAP courses but there are leaks oo some internet pages with this content, you can even find instructors material. For old material is easy to find.