We were multiorganismal before we were multicellular.— Angela Douglas
Horseshoe Crabs and Velvet Worms (which is subtitled "The Story of the Animals and Plants That Time Has Left Behind") by Richard Fortey is a book I read earlier this year and found very interesting. The author is very careful to not call the things he discusses "living fossils" because he would like you to know that there is every likelihood these life forms have been and continue to evolve, but just that they have stayed in the same niche and kept the same appearance.
One of the more interesting, to me, life forms discussed in the book are stromatolites. These are not animals and not plants. Nor are they fungi. They are entirely (or largely) cyanobacteria. And as prokaryotes (no cell nucleus) they are very distant cousins from us animals and plants.
Stromatolites fit the quote above quite well. Huge structures created by billions of single cell organisms working together for thousands of years. Biofilms of cyanobacteria slowly grow these rock-like formations that can be found in multi-billion year old fossils and in a few remote places of modern Earth.
"Some [of the living] structures are pillars up to 1.5 metres (5 ft) high and have taken thousands of years to grow. In the Marble Bar area of Western Australia there are fossil stromatolites approximately 50 metres high and 30 metres diameter. These are estimated to be over 3 billion years old. Typical growth is about 0.5 mm per year."
50 meters at half a millimeter a year is 100,000 years. Those are the engines of oxygen production of Archean Era.
Many of the other life forms in the book are quite interesting too.
By comparison the velvet worms, dating back to the Cambrian, are positive newcomers on the scene. These creatures are very primitive segmented animals. Once you've got the genes to make a thing, increasing your body size by making more of a that thing is relatively simple. So it is with velvet worms, which have body structure that can be mentally imagined by crossing an earthworm with a centipede. They have between a dozen and four dozen segments, move very slowly and quietly, to ambush prey on anything they can find that's the right size to eat. Originally they did this underwater, but are now only known on land, albeit only in humid environments.
Horseshoe crabs are an often cited "living fossil" and need little introduction. Ginkgos, ferns, hagfish are some of the other organisms discussed in the book. The author limits himself to things he can visit in nature, so sea life is not well represented. Hagfish, which lacking a jaw are not true fish, are readily caught and thus observable. (Not that hagfish are intended target of fishermen, they are instead an undesirably by-catch that seemingly finds large trawl nets full of fish an enticing feeding opportunity.)
Books of this ilk are exactly what I look for.