Recommended China Guzheng Grade Examination Results compiled by Shanghai Guzheng Association. I am using this version myself.
The most popular should still be the basic collection of guzheng of the Central Conservatory of Music.
New Rhyme of Guzheng (Shanghai Conservatory of Music Press, 2006) is a guzheng textbook edited by Mr. Wang Gangqiang, a famous guzheng expert in Zhejiang Province. The level should be suitable for you, including famous songs, which are more innovative.
This is a relatively complete version of Guzheng Performance Course (Part I and Part II) edited by Professor Lin Ling of China Conservatory of Music (Baihua Literature and Art Publishing House, 2008).
Zheng (Central Music Press, 2004), edited by Tommy, is a relatively formal version, but there are only soundtracks.
Wang Zhongshan's Collection of National Guzheng Performance Examination Works (Anhui Literature and Art Publishing House, 2008) was compiled by a generation of famous artists, which probably caters to personal interests and is graded.
I won't recommend the primer, and you probably won't need it.
If you have any questions, please refer to here.