You can find Stuart's books on various retailer sites, including:
Don't see the book you want in your preferred store? You might try via Stuart's Books2Read profile.
If you're an Amazon customer, you might be interested in Kindle Unlimited, which gives you an "all you can read" service for a low monthly price. Some books by Stuart J. Whitmore may be available via Kindle Unlimited, in which case you can read them for free -- that is, at no additional cost beyond your Kindle Unlimited subscription.
It's also possible to request that your local library add books by Stuart J. Whitmore to their collection so that you can check them out for free. For now, the odds are fairly low but it doesn't hurt to ask! Success is more likely to happen for e-books than paperbacks, especially if your local library uses OverDrive.