Finding Tamil novels as proper ebooks took me forever compared to other languages. The market's smaller, so legitimate free downloads are scarce. I've had luck with public domain works from sites like Project Gutenberg Tamil – they've got classics by Kalki and others in PDF. For newer stuff, some authors release first books in series for free on their blogs or sites like Smashwords as a sampler.
Honestly, most 'free ebook' searches lead to sketchy aggregators with pirated EPUBs. The formatting's usually terrible, with missing diacritics that ruin the Tamil script. I'd rather read a clean web version than a broken download. A better route is checking if your local library offers Tamil collections through Libby or OverDrive. Mine doesn't, but a friend in a bigger city gets access that way.
Sometimes the official publisher's site, like Vikatan or Vanathi, puts up sample chapters as PDFs you can save. It's not the whole book, but it's legal and supports the author. I've built a small collection that way, then decided to buy the full ebook if the sample hooked me. The download quest often ends with realizing paid platforms like Amazon or Google Books have the most reliable EPUB selection, even if free picks are limited.