Tome- a large heavy book, especially one dealing with a serious topic. (Oxford Advanced Learner's Dictionary)
Tome-any book; especially a large or scholarly book. ( The American Heritage Dictionary of the English Language, College Edition)
I prefer the Oxford Advanced Learner's Dictionary definition. A tome qualifies to be a book of mighty length. When I read a tome, it must be a topic that fixates me. Most books I read exceed 200 pages yet I measure a tome at 400 pages and above.
Twenty tomes I've read thus far:
1) A Life in Letters by Zora Neale Hurston, collected and edited by Carla Kaplan. 912 pages
2) Mick Jagger by Philip Norman. 640 pages
3) Meet Me in the Bathroom: Rebirth and Rock and Roll in New York City 2001-2011 by Lizzy Goodman. 622 pages
4) Mystic River: a novel by Dennis Lehane. 592 pages
5) Working: People Talk About What They Do All Day and How They Feel About What They Do by Studs Terkel. 589 pages
6) Girls Like Us: Carole King, Joni Mitchell, and Carly Simon - and the Journey of a Generation by Sheila Weller. 584 pages
7) Mother of Believers by Kamran Pasha. 560 pages
8) Gig: Americans Talk About Their Jobs at the Turn of the Millennium edited by John Bowe, Marisa Bowe, and Sabin Streeter. 560 pages
9) The Years with Laura Diaz: a novel by Carlos Fuentes. 528 pages
10) Jubilee by Margaret Walker. 497 pages
11) A Piece of Cake: a memoir by Cupcake Brown. 472 pages
12) The House of the Spirits by Isabel Allende. 448 pages
13) Spinning Straw into Gold: What Fairy Tales Reveal About the Transformations in a Woman's Life by Joan Gould. 448 pages
14) Caramelo by Sandra Cisneros. 439 pages
15) It's All About Love: Black Writers on Soul Mates, Family and Friends by Marita Golden. 432 pages
16) The Coldest Winter Ever by Sister Souljah. 430 pages
17) Life Notes: Personal Writings by Contemporary Black Women by Patricia Bell-Scott. 429 pages
18) Peach Blossom Pavilion by Mingmei Yip. 421 pages
19) Big Sex Little Death: a memoir by Susie Bright. 412 pages
20) Kite Strings of the Southern Cross by Laurie Gough. 400 pages
Feel free to share your opinion. Present books that engaged you. I might read them.
Cheers!!
postscript: I read hardcover and softcover formats exclusively. Keep this in mind when disclosing your recommendations.