Challenge: Older Books

(This post is part of the Wednesday Weekly Blogging Challenge. You can find links to other writers' answers over at Long and Short Reviews.)

Prompt: Older books that more people should read.

My immediate answer for this one is incredibly easy: the Darwath series by Barbara Hambly. 

The series consists of a central trilogy and two follow-up books; there are also a couple of short stories available from the author's website. The series is a portal fantasy, at least partly; two of the main characters are from modern-day Earth. (The series is old enough that it might not exactly be modern-day; but Earth in the 1980s, and it was modern enough when the books were written.) However, the characters from the other side of the portal, the realm of Darwath, are every bit as well-drawn and essential to the plot as the ones from Earth. 

The series has some of the most incredible world-building I've ever encountered, and some of the moments of revelation that happen as the characters learn more about their world are positively stunning. The best way I can think to describe it is that it's sort of Lord of the Rings meets H.P. Lovecraft, as written by a History major with experience in martial arts. 

I've re-read the entire series probably a dozen times, and each time I come away amazed.