A far-from-comprehensive list of authors on Tumblr:

(all authors listed in alphabetical order by last name)

Rachel Alexander // @therkalexander

Laurie Halse Anderson // @lauriehalseanderson

Victoria Aveyard // @vaveyard

Cassandra Clare // @cassandraclare

Zoraida Córdova // @wanderlands

Neil Gaiman // @neil-gaiman

Malinda Lo // @malindalo

Tamsyn Muir // @tazmuir

Samantha Shannon // @sshannonauthor

please let me know who I am missing!

for the sake of brevity (and my sanity) this list is limited to published authors (ie. does not include aspiring authors which you can find using hashtags like #writeblr or #WIP or #my-writing and so on and so forth).