Lu Zongda's "A General Theory of Shuowen Jiezi" (Beijing Publishing House 198 1) expounds the style of the book.
Zhang Shunhui's A Guide to the Interpretation of Chinese Characters, Bashu Bookstore 1990, China International Broadcasting Press, 2008. I think this book reflects Mr. Zhang's personal opinion, but the introduction to the original work is rather general.
Zhang Zhu's How to Learn to Interpret Chinese Characters (Henan People's Publishing House 1988) introduces the basic situation and style of interpreting Chinese characters.
Su Baorong's Notes on Interpretation of Ci (Shaanxi People's Publishing House, 2000) selects some commonly used and popular words for translation, which is helpful to understand the original text.
Yin's Intensive Reading of Shuowen Jiezi (Fudan University Press, 2006) mainly explains radicals and some commonly used words, which makes me feel very general.
Personally, I think Yao Xiaosui, Lu Zongda and Zhang are relatively studious, and they completely introduced the style of the book. However, reading Shuowen can't just rely on theory, but must read the original. If you don't understand, you can consult Su Baorong's book or Tang's Modern Translation of Shuowen Jiezi (Yuelu Bookstore). Further research should refer to Duan Yucai's Notes on Explaining Words. Radicals are the key to understand the structure of Chinese characters, and it is best to make clear the sound and meaning of radicals. 540 radicals are the raw materials of all Chinese characters. Radicals can be found in Zou Xiaoli's Interpretation of the Origin of Basic Chinese Characters, Xu Fu's Positive Solution of Shuowen Radicals, Dong Lianchi's Interpretation of Shuowen Radicals, New Evidence of Shuowen Radicals, and Zuo Min 'an's Interpretation of Shuowen Radicals (the above books are easier to understand than Zou Xiaoli's, but the radicals in the books are incomplete and only common. )