Unfortunately, you can't currently read a full book by Brenda Jackson online for free. There are methods that may work, such as searching for Bit Torrent copies of the books, but this is illegal and as such I wouldn't recommend it!
What's the cheapest way to read a Brenda Jackson book?
Although there aren't any free online copies of books by Brenda Jackson, there are some options that won't break the bank. If you have a Kindle you're likely to find a better deal, for example, and Amazon currently sell Kindle versions of Zane by Brenda Jackson for 98p!
You can expect to pay around £5 currently on Amazon for a paperback book by Brenda Jackson, which isn't too bad in today's market.
Are there any other options?
If you're looking for cheap books, I would always recommend checking your local charity shops.
If you're lucky, you'll be able to find a copy of a Brenda Jackson book for a really low price. The other alternative is to look at eBay and see whether anyone is having a clear out. There's always plenty of bargains to be found on there!