Having been an avid user of WordPress for more than 10 years, I’ve come to know the products that help your WordPress site / business and which, frankly, don’t. The following is a list of products I highly recommend and am, in fact, using myself:
Gravity Forms
The most complete and versatile WordPress Form plugin
Yoast SEO
Optimization of your WordPress sites is only done proper with Yoast SEO
Codeable
If you need help in small projects, you’ll need Codeable. They’re the best.
Books About WordPress that I recommend
There are lot of books written about WordPress, but unfortunately, not all of them are that great. These however are: