There has definitely been some controversy surrounding readable books being online. As far as resources, I think this is a great idea, especially when researching a tough topic. You are not always able to find the certain text you are looking for through the local library, or their ordering system. As well, your budget may be flexible to include books for research or school. So, for these purposes, I think that being able to access old or uncommon books online is a wonderful resource to anyone!
I was looking for some resources last week, and on my search came across two excellent sites. One you might know already is Google Books, however, some of you may not know about the Advanced Search page where you can search for solely free titles. Here, next to the heading "search" select "full view only" and your search will include complete books! (A side note, many of the 'limited preview" books are only missing a few pages and can also be useful)
What if you cannot get to the local library? try Open Library!! Different ways that you can obtain books either through borrowing ebooks, borrowing real books or it gives links to major websites that carrys the book so you can purchase it! Too many books to imagine, and well laid out too!
The last site I happened upon was the Online Books Page which uses links to other various websites that either has the book transcribed or copies of each page. For example, perhaps you want to learn more about Florence Nightingale, and read the book Florence Nightingale: War Nurse or, read about Montessori in Maria Montessori's book "The Montessori Method". You can do this, through the Online Books website! It has a simple setup, but ultimately its the information you're looking for!