Theo's Odyssey

It's only 425g but it's packed with weighty theosophical ideas! Theo's Odyssey by Catherine Clement is a book that's aimed at young readers but is a really good way for readers of any age to get a feel for many different belief systems. And it's not so heavy you can't read it in bed. Bonus.

TS Spivet

Awesome! Another book with footnotes and maps and stuff!
My copy of "The Selected Works of T. S. Spivet" by Reif Larsen is ISBN 9781846552786, weighing in at 0.975kg.
Because it's quite large format and paperback, it's not really a "read on the bus" kind of thing. More of a "savour at home" arrangement.
Impress your partner and cat. Grunt each time you pick it up and go "phew - just gonna read this awesome book again!"
The story is actually pretty cool, and slightly reminiscent of "The Curious Incident of the Dog in the Night-time".

Jordan/Sanderson

Crikey the new Robert Jordan ghost-written Gathering Storm is a mighty beast! Brandon Sanderson has obviously studied Jordan's previous work well, and no doubt there are about six hundred separate references to an Aes Sedai twitching her shawl. Why did I ever stop reading that series..? Anyway, I promise to get my scales set up very soon so you get some accurate weight information. This crucial research can be delayed no longer.

Inaugural, unscientific posting

I don't have my precision book-weighing scales set up just yet, so I'll start by just saying that I read Neal Stephenson's Anathem in Trade Paperback (TP) format (ISBN 9781843549154) a while ago and it was pretty hefty. Imagine if I'd bought the Hardback (HB) version! It was 800 pages of creative intelligence that I felt just on the verge of understanding. See http://www.fantasticfiction.co.uk/s/neal-stephenson/anathem.htm
 I also loved Stephenson's Baroque Cycle books because I enjoy footnotes in fiction works.