This is a list of resources you can use to improve your preservation gameplay, some of them are more advanced than others and some require way more effort to understand but all of them contain usefull information that you can apply to be a better player
Wyrmrest Temple is currently the main hub for high level Preservation theorycrafting and gameplay, there's a lot of high level players active at all times of day to answer questions and help you with any issue you might have. Most of the people that maintain the guides and run the numbers on everything preservation related are active in there.
Maintained by Voulk, QE is the best source for general healer information and hosts the QELive tool for healer simulations. Here you will find articles with high level knowledge about upcoming content and QELive will be your main tool when you need help picking between pieces of gear or choosing great vault rewards.
Also by Voulk, the Wowhead guide is always kept up to date with the latest on talent builds, stat priorities and bis crafted items. Great resource to quickly confirm everything about your set up is right and check out our current meta builds.
Similar to the Wowhead guide but maintained by Myth. Always up to date with talents, stats, bis lists and gameplay tips.
Maintained by Total, the spell reference document is a bit of a heavy read but is full of incredibly detailed and usefull information about every single spell and talent of the spec