I have personally used the following resources and have found them to be very helpful and effective. I would highly recommend them to anyone who is looking for help to feel calm and centred.

I have no association with any of the companies and do not benefit in any way from you purchasing them.

Clear, Calm and Confident - How to change your life in 30 days by Christian Dunham. 

This short easy to read book comes with a cd which contains a superb hypnotherapy track of about 30 minutes. I have used it many times to help feel really calm and would highly recommend it.

Clear, Calm & Confident – CHRISTIAN DUNHAM 

Experience Yoga Nidra - Guided Deep Relaxation by Swami Janakanada

This is an amazingly good guided Yoga Nidra. It really is superb, a master piece. The CD / audio contains a short version of about 20 mins and a longer version of about 50 minutes. It is impossible not to feel calmer and uplifted after listening to this. It's basically yoga and deep relaxation for the mind. Search online and you'll find lots of options to purchase. 

Pryanayama - either 'Universal Breathing' or 'Health through breathing' by Saagara (App developers)

In western cultures we often pay little attention to breathing well and yet how we breath has a profound affect on our emotional state, our blood pressure, our digestion and many other functions. These simple apps (available on the App Store) show you how to breath well, using your diaphragm (instead of your chest). It includes an easy to follow visual and several levels (beginner, intermediate and advanced). By using this App for 10 minutes, it is impossible not to feel calmer. Highly recommend this for daily use. Lots about this also on the web in other formats.

Any hypnotherapy audios by Dr Loretta Siani - available via Audible.com and the ITunes Store

These are again exquisitely produced hypnotherapy tracks. I use them regularly. Favourites include: 'In the Middle of Nowhere' and 'The Lake'. They are each 30 minutes long and only cost a few pounds. Dr Siani's voice, her tone and tempo, the tension and shape of the hypnotherapy stories are superb. Very subtle background sounds are really well done.