The first book that I always recommend to people, is Letters of Light, by Rabbi Aaron L Raskin
Rabbi Raskin has done an amazing job of taking the esoteric nature of the Aleph Bet, and making it accessible to those who have never encountered this language.
What I appreciated most about this book, was not only the amazing content, but how Rabbi Raskin provides a brilliant introduction to the vast esoteric nature of each of the letters, and provides a comfortable platform from which to launch a much deeper conversation.
This is the book that opened me to a world of understanding and learning that has been passionately pursued and taught among Jewish communities for thousands of years, and I can't stress enough just how important what the Rabbis's teach on this subject is to humanity.
This book is available through Amazon in Harback and Kindle versions. The Hardback version can be difficult to get a hold of if you are outside the USA.
The next book I recommend is, The Wisdom in the Hewbrew Alphabet, by Rabbi Michael L Munk.
Often I find it difficult to describe what type of book this is. So in this instance I will directly quote from the jacket sleeve
"The alphabet - if correctly understood - is a primer for life. Ethical conduct, religious guidance, philisophical insights, all are nested in the curls, crowns and combinations of the Hebrew letters"
Simply put, this book delves into the over reaching influence that the living nature of YHVH's voice, as encapsulated in the living letters, has over the Jewsish mindset, and therefore in my opinion over us all.
This book is only available in Hardback via Amazon, that I am aware of.
The resaon that I recommend the above books, is that I have found that they are the best resources that enable you to learn design, meaning and gematria. All three are foundational to engaging with the letters on a deeper level, and I have found that many authors will focus on one thing and not provide enough resource for the others.