Maybe you were a fan of the old books and unsure of the new material. Maybe you just discovered there was more material then just the Star Wars movies/TV shows. Either way, if you've decided to finally look into the new Star Wars expanded universe, here are my recommends to start with:
If you are an original trilogy fan:
Lords of the Sith by Paul S. Kemp
Interested in learning more about what Vader's time before Luke showed up was like? Try this novel, about the Emperor and Vader getting stranded on a planet together.
Tarkin by James Luceno
If you were ever interested in the dude who got offed by Vader that one time, here's an entire book looking at his life and how he came to be part of the Empire.
Lost Stars by Claudia Gray
If you want to know more about life under Empire rule, I highly recommend this book by Claudia Gray, who I think has captured Star Wars the best in her novels.
Star Wars New Canon Graphic Novels
For original trilogy fans, I highly recommend this series, which takes place between A New Hope and The Empire Strikes Back.
If you actually are really into Clone Wars and Rebels:
A New Dawn by John Jackson Miller
If you love Kanan, this book is for you. It goes into what Kanan got up too after the Jedi fell and before the start of Rebels.
Ahsoka by E. K. Johnston
Love Ahsoka? Me too. I highly recommend this book that shows what she got up too after we see her in Clone Wars and before we see her Rebels.
Kanan: First Blood graphic novel
For those who are curious about what Kanan's Padawan years were like.
If you really loved The Force Awakens:
Bloodline by Claudia Gray
Yes my friends, a Princess Leia centered book. It is fabulous and gives great plausibility for how the First Order were able to arise again after everyone lived through the Empire.
Poe Dameron graphic novel series
Yes, Poe has his own graphic novel series. Dreams do come true.
You just saw Rogue One:
Rogue One: Catalyst by James Luceno
Sadly, I have still not seen Rogue One, and probably won't till it comes to DVD/Blu-Ray. So I have not yet read this one. I've heard good things about it though.