Unfortunately, 20,000 words isn’t typically considered a book-length novel.  There are a number of answers to this question on Quora.

The traditional rule of thumb, which still pretty much applies to an adult trade novel, is: between 80,000–120,000 words, or 320–480 pages. A typical page holds 250 words in 12 point Times New Roman, double spaced with one-inch margins. Some novels can be shorter at 50,000 words (really a novella), or 200 pages.

Anything over 480 pages may flag the work as needing to be edited; below 50,000 words can indicate the story has been rushed or not adequately fleshed out. A short book is called a novella–that is between 30,000-60,000 words. Historically, these standard lengths are based on the space required to develop an adequate story arc, reader expectations and—especially in the old days—upon the publisher’s printing and budgetary constraints. Digital publishing has made this third issue a little less important. Still. digital books cost the publisher per page. Some genre or category books might run toward a short count. For example, thrillers sometimes clock in at about 60,000-65,000 words.

Rules are made to be broken, however. There are always exceptions. If you are a new writer, it helps your publishing chances to stick to tried and true guidelines.

Here’s a good guide for story length:
Short stories: 15,000-30,000 words (50-100 pages)
Novellas: 30,000-60,000 words (100-200 pages)
Novels: 80,000-120,0000 words (200-400 pages) and beyond