A step-by-step guide for users to learn how to use ABBYY finereader for performing optical character recognition (OCR) on a text corpus.

A step-by-step guide for users to learn how to use Tesseract open-source software for performing optical character recognition (OCR) on a text corpus.

Tutorial for using optical character recognition (OCR) on the Google Cloud Platform

University of Illinois' Introduction to OCR and Searchable PDFs, with several how-to guides for popular software packages (ABBYY, Tesseract, Acrobat)

State of the field report from 2018, offering research agenda for working with historical and multilingual OCR projects

A gentle yet theoretical and practical discussion of what happens when performing OCR

The Internet Archive provides plenty of historical materials that have already been OCRed. Check here to see if your materials have already undergone OCR before trying to do this yourself.

Adam Matthews (a database vendor) has been performing handwriting OCR using their in-house engine. Not all their materials are available yet but several major databases are.