Online Reference Books

Literary Resources on the Internet
You will find a growing number of reference books are now available online. This means there is not as much need to visit libraries and other institutions to undertake research. Many of these premises have restricted access so consulting these online libraries is often the only way to discover new information.
The types of publications you can read include directories, historical guides, business records, and personal stories.
Many universities and other organisations have digitised records and books to be viewed, and sometimes searched on and downloaded, online. Even institutions abroad may carry UK-relevant titles. These may be complete digitised books or just their indices.
The UK has a well-connected library service. The British Library has a vast index of titles, most of which you can access at the British Library Reading Rooms.
Upenn Books Page
The resource I enjoy using the most is probably The University of Pennsylvania in the USA, which claims to list over 3 million books online. Their website not only gives you access to these but also lists many other resources.
The HathiTrust
The HathiTrust was founded in 2008 as a not-for-profit collaborative of academic and research libraries now preserving 18+ million digitized items in the HathiTrust Digital Library. You don’t need to log in or have an account to search in HathiTrust, and up to 40% of the collection is available for anyone to read online.
Google Books
You may not be surprised to know that Google has “the world’s most comprehensive index of full-text books.” Using a simple Google account (a Gmail account for instance) you are able to create your own library of Genealogical resources.
Project Gutenberg

I was really surprised when I realised how many reference books were available on the Internet.