Imam Rassi Society has kindly forwarded to us the translations of credal statements and Imamate statements by Imam al Hadi. Here is a Sunni scholar's description of Imam al Hadi's role in the development of Zaidism (see my post "Zaidi books of Doctrine" for full description).
"During the time of Imam Ali alayhis 'salam calipate the people of Yemen stayed loyal to Imam Ali alayhis 'salam and his sons. However, after this era the tribes of Yemen did not show much oppositions against the Ummavi rulers. Then after a few centuries the people of Yemen decided to invite one of the Aalids to rule over them. Imam Hadi alayhis 'salam accepted the invitition and arrived in Yemen. He stayed there and also established an imamate rule which managed that last up until the mid 20th century.
Similar to his grandfather, Imam Rassi alayhis 'salam, he was a very prolific writer who wrote works on theology (kalaam), fiqh, tafsir, language, history, dua, and poetry. He also issued fatwas and answered many questions that was sent to him by people."
Imam al Haadi was the Imam from 898 till 911.
To read some of Imam al Haadi's work, click on the following link: