Here's the list/spreadsheet of titles they are seeking; it will be updated with titles they've received scans for ("Pending") and those which have been accepted ("Submitted"). There are tabs for OD&D, 1E, 2E, Dragonlance Fifth Age, 3E, and 3.5. If you have one of these titles and want to send in a scan you need to fill out this form.
The $50 is added as credit to your DTRPG account, and you can cash it out via PayPal or you can spend it in store.
More info? Click here.
The main problem is that the prescribed scanning process involves you destroying your book (and that doesn't even guarantee they'll accept it). DTRPG asks you to cut off the spine and use a sheet-fed scanner. Given the classic status of older titles, the book itself may be worth more than $50 to you. That said, a lot of them can be found on eBay for reasonable prices, so it may be worth it. I, personally, am incapable of destroying, selling, or losing a book, but that's just me.