Founded in 2008, HathiTrust is a not-for-profit collaborative of research libraries preserving 17+ million digitized items. HathiTrust offers full reading access if allowable by U.S. copyright law (Full View). For us most valuable is a lot of the standard dog literature from bygone times, some selected stud books (AKC, FDSB, KCSB,…) and also a lot of periodical papers.
As the copyrights expire over the years, you should search at least once a year for new additions.
Link to the HathiTrust Digital Library
You can download only single pages, which is rather time consuming if you want to download a complete book. A great tool for downloading a complete book with just one click is the Hathi Download Helper which you can download here.