Finished Eisenhorn Trilogy, looking for Space Marines books recommendations

Hey everyone, I'm new to the Warhammer 40k universe and just finished the Eisenhorn trilogy, which I really enjoyed! While I understand the next logical step would be to continue with the Ravenor trilogy, I want to take a little break and explore some Space Marines stories, as they're what attract me most about the 40k universe. I had Helsreach and the Ultramarines books in mind, but I'm not entirely sure. Could you recommend any good Space Marines books for someone who's still relatively new to the lore? Thanks in advance!