I am trying to find some commentaries on Herodotus’ language on the Net, but I have not yet found anything. So, I would like to know if there is anyone who knows some books available on the Net. Which ones would be the best for you? I cannot afford a purchase of a book from Amazon.
You could check Geoffrey Steadman’s site. He has commentaries in PDF format he offers for free. There’s a very short introduction on the language and a Greek text with running vocabulary. https://geoffreysteadman.com
Personally I have used and profited from works already mentioned by Paul: Geoffrey Steadman’s works on Herodotus 1 and Herodotus 7, George A. Sheets’ Bryn Mawr commentary on Herodotus Book I, Stephen T. Newmyer’s Bryn Mawr commentary on Herodotus Book III, How and Wells’ commentary. Also helpful to me were notes in E. S. Shuckburgh’s editions from the late 19th century (at archive.net). I believe Shuckburgh treats each of the books of Herodotus in a separate edition, but I don’t know whether all of Shuckburgh’s editions are available.
Each of the following threads on Textkit discuss Herodotus commentaries, however briefly. Not knowing at what level your Ancient Greek is, nor whether you know German (for some of the commentaries mentioned in these threads are in German), I leave it to you to sift through these threads, or not.