There are a few "tricks". Larry is right, the bottom line is you have to OCR the text. And then proof that text against the original. The "trick" is the tools you use. Even with good tools, it's time consuming. With bad tools, it's unmanageable long term.
For the text I digitized, I bought the books myself and scanned them to get a higher resolution scan. Archive.org scans are ok for reading, but for digitizing text, you want a higher resolution scan IF the text is small (like with a commentary--not so much a 75 page devotional book with large print). A PDF scan is just a digital picture of each page. The better picture you take (high res scan) the easier the text will be to interpret.
The text interpretation is best done, in my experience, with Abby Finereader Pro. It's the most efficient with the least errors. I like how you can go page by page in a split screen mode, with the original PDF page on the left side and the digitized text on the right side. Words that Abby is uncertain of, or words that don't match a spell check are highlighted for your review. You can click a word in the digitized half of the screen, and that word's location is highlighted on the PDF side of the screen.
That's the trick: 1) scan resolution and 2) decent OCR software. You can do it without either, it's just a question of time--how much time it's going to take.
But isn't the bottom line that you end up re-typing the whole book into a word processing program? If you're re-typing everything to get an electronic copy, then the quality of your OCR software doesn't seem all that important to me. Or are you saying that the OCR image becomes the e-Sword module? I'm obviously missing some fundamental point.