Hands down the best book about mermaids for an informative, mostly folkloric approach is a long out-of-print book first published in 1961, Sea Enchantress the Tale of the Mermaid and Her Kin by Gwen Benwell and Sir Arthur Waugh. A second edition was issued a few years later. I haven't seen that one but I haven't seen any description stating it is expanded. Used copies of either edition are rare and expensive and I acquired one during the process of compiling my own Mermaid and Other Water Spirit Tales From Around the World.
My book is more anthology than discussion although the nine articles within it provide ample discussion, much of which is similar to what you can find in much of Sea Enchantress the Tale of the Mermaid and Her Kin. Sea Enchantress does not include tales but it discusses and lists many of them. And many of the sources used for Sea Enchantress are provided full text in Mermaid and Other Water Spirit Tales From Around the World. In Sea Enchantress the focus is on European lore as well as lore from "New World," in other words, the Americas. Africa and Asia are mostly neglected.
Overall, my book and Sea Enchantress are great companion books, enhancing and complementing each other. Mine is 827 pages long. Sea Enchantress is 287 pages. That's what including over 150 tales will do to your page length.
I didn't acquire my own copy of Sea Enchantress the Tale of the Mermaid and Her Kin until my very last week of working on my own book. I was pleased to discover I had compiled many of the same resources as well as some extras so although I realized I had recreated some of the work, I had been on the right track and hadn't missed any major sources. The greatest advantage for a mermaid enthusiast in acquiring Sea Enchantress would be for the discussion of 20th century lore (at least until the 1960s when it was published) since my book is comprised almost entirely of 19th century and earlier.
Here is the Table of Contents from Sea Enchantress the Tale of the Mermaid and Her Kin. (The table of contents for Mermaid and Other Water Spirit Tales From Around the World is available here.)
1. Origin: Myth or Mammal?
2. In the Ancient World
3. In Classical Times
4. In the Early Christian Era
5. In the Middle Ages
6. From the Elizabethan Era to the Eighteenth Century
7. In the Nineteenth Century
8. The Mermaid in Church
9. In the Folklore of the British Isles: England, Wales and Ireland
10. In the Folklore of the British Isles: Scotland
11. In the Folklore Overseas: In the Old World
12. In Folklore Overseas: In the New World
13. In Heraldry, Sign, Map, Coinage and Philately
14. In Literature
15. In Our Day