Text analysis and text mining are processes that derive information from texts such as novels, monographs, articles, web pages, etc. You can use text analysis tools to quickly search through a large corpus, to generate word clouds or find word frequency, to more complex tasks like identifying patterns in parts of speech or identifying sentiments, moods, and emotions in a corpus.
Voyant is a web-based tool for reading and analyzing your digital texts. It can use texts in a variety of formats including plain text, HTML, XML, PDF, RTF and MS Word. Use it to perform lexical analysis including the study of frequency and distribution data; in particular.
TAPoR, or the Text Analysis Portal for Research, is an online portal where users can store and keep track of texts they wish to study, learn about and experiment with different tools, and use those tools to analyze text.
See the Project Bamboo DiRT Tools page on text mining for many more tools.