This is a very good question! One that many authors struggle with and one I believe we can help you address in a successful way. As you may know, people are bombarded daily with requests to join newsletters, blogs and more. You will need to capture the attention of your target market by providing them with something of GREAT VALUE. This great value item must stand out as unique. Your invitation to join must make them go WOW!
Signing up to join your newsletter or to receive your updates is not a solid or good enough reason to convince them to give you their email. Not in today’s world. Teasers with chapters don't do it either as readers can simply go to Amazon and Smashwords to download free sample chapters directly from there of your books. In fact, readers can go and find hundreds of free novels and books in Amazon’s free section for any genre.
Some authors are offering not one, but two novels to convince readers to join their mailing list. This helps them to stand out from the crowd.
If you only have one book in the non-fiction genre, then consider writing a short book related to the same topic. The book could offer tips, do's and don'ts or top mistakes to avoid in your niche.
If you are in the fiction genre, then consider writing a novella around 20-25 thousand words to boost your email subscription rate. Until you do that you can offer sample chapters, however, the best return will be achieved when you offer a novella or a novel. If you are writing a series, then consider offering the first book in the series for free to build your email list. Thus, when your second book is ready to be released you already have an email list of targeted readers who will support you and buy your second book.

Please note that your free book needs to be the best you can offer; it must be of high quality that shows your style of writing and what your book is about. It must be proofread by a professional [typos and grammatical errors will send your reputation downhill very quickly and people will unsubscribe from your newsletter for this reason alone]. You will not have a second chance with these people. Add to that your book cover must be of good quality and not a DIY cover. [You can get one done for under $50 at Fiverr.com]
It is extremely important to identify your target market clearly so you can promote your free eBook directly to them online so your time and efforts don't get wasted.

Here are some ways to promote your free eBook to build your list:

1 Inside your published books on Amazon and online retailers:
Add the FREE eBook cover at the beginning of your book and a big 'FREE DOWNLOAD' with a link to the Landing Page of your Free eBook
This could be added after the Copyright notice and before Table of Contents
Do the same at the end of the book.

2 Website and blog:
Create a landing page for your free eBook on your website. Make it visible to visitors

3 Promote on Twitter:
Make the pinned tweet your free book. Send traffic from Twitter to your landing page.
Post a tweet once a day at different times. Keeping in mind you need to share other useful content and not just promotional tweets.

4 Promote on Facebook:
If you have a Facebook page, then pin your free eBook to the page
Organize a giveaway
Run a Facebook ad lead generation campaign offering your free book to generate targeted subscribers.

Join Facebook groups and promote your free book
Create your own Facebook group once you have followers and promote your free book, newsletter, and more there.

5 Other Social Media channels, promote your free book on any platform where you have an account
Please note: social media promotion needs to be done consistently and regularly -
Create social media banners promoting your free book. This will include the book cover with a Compelling headline and a Call to Action. You can see many examples at Facebook or Twitter

6 Join Copromote.com and boost your tweet or Facebook post

7 Join Instafreebie.com and create a giveaway for your free book.

This is a good start for building your email list for your newsletter. However, to help you with other options, I'd like to provide you with a complimentary copy of my recently released e-book 'The Self-Publishing Tools of Trade Every Author Must Know'. This is an immense book of tips, resources, and knowledge that will save you months of trial and error in your self-publishing journey. The eBook includes information on self-publishing, author branding, and eBook marketing. For your free copy to download, go to:
http://xanapublishingandmarketing.com/free-ebook-3/

Remember, it does take time and focused planning to build a target list of readers in your genre, however, if you're willing to put in the time and the effort, you can absolutely reap the rewards. I hope this helps. Please feel free to ask questions, I will be more than happy to help guide you.