We decide which books to index based on our knowledge (from many years of doing this) whether the book is likely to appeal to EYB members and whether previous books from that author or on that subject have been popular. Lastly we also need the publisher to supply us with the book for indexing - digital or print - or whether an indexer can get it from their library - our budget does not stretch to us buying the books we index!
Happy to hear those recent indexes met your approval!