I used “LPIC-1 Sicher zur erfolgreichen Linux-Zertifizierung” by Harald Maaßen, published by Rheinwerk Computing (in German), ISBN 978-3-8362-6375-7,v and I liked it quite a lot.
There’s quite a few of these books, also in English, and they’re all kinda useful.
I work at a large scientific library, so I used it quite a lot to get me a selection of LPIC books. Also, I usually not just read the book, but I had my computer with me, so I could a) try things out that were discussed in the book and b) read up the whole (!) man page of any command used in the book. I found that that helped me quite a lot, because it gives you an opportunity to tinker and practice.
All the best for your own certification!
Mein Eindruck war, dass die LPIC generell ein Stück hinterher hinkt, was so modernes Linux angeht, aber den Rest hat man sich auch schnell mal online angelesen. So Systemd,
ip,ss, …