I can't be particularly rational or analytic about this book. It's a childhood favorite, that appeared in our house when I was maybe 10 or so in odd edition dated 1935 that lacked an author credit.
Re-reading it as adult, I'm struck by either how much it was perfect for my tastes or, maybe, how much it helped shape my tastes. Mainly, I think, for how much awesome, extraneous stuff it has in it. I'm typing this with my back to a map of Westeros on my wall and I think the sort of intriguing but often unexplained side details crammed into this book are definitely a thing I like. My lovely wife is pretty much anti that stuff and prefers a more streamlined narrative, I think it's fair to say, but I delight in things like Robin being treated by blood-letting and his men all being afraid of the Spectre Mare, which are plot points, but not really explained, as well as other details in the book that aren't even that much, like the widespread belief in witches and superstition people constantly express and the writer's salting the text with old English terms sometimes explained in parenthesis and sometimes not.
It's told in episodes and even with them, it's not a straight-line narrative, with a lot of little detours and derails that are there, seemingly, just for fun. And other places, it's refreshingly direct, such as where Robin decides the sheriff of Nottingham has crossed the line and shoots him dead. This is a more violent, wilder Robin Hood whose men try to burn Guy of Gisborne in his house and who doesn't hesitate to hang enemies get gets ahold of. The climatic scene where Robin leads an attack on his enemies castle (subtly nicknamed The Evil Hold) ends with him capturing and hanging the lords of the place, rather than killing them in a sword fight, for instance. It also deflates the standard scene where Richard I meets Robin and pardons him by noting that most of his former outlaws go often to campaign with the Lionhearted in France and end up dead. Easy to see how liking this stuff at 10 leads A Song of Ice Fire mega fandom at 36. And there are brownies who help Robin. Also, non-standard for these things is Robin helping Jews fleeing pogroms.
The book's also fascinating now for its politics. By the amount of anti-rich sentiment and calls for literal class warfare (which often gets soft-pedaled in late 20th century Robin Hoods) I wouldn't be shocked if Mr. Gilbert voted for Eugene V. Debs. It's also super, super Catholic, with Robin namedropping Our Lady pretty much every page. I guess it's also of its still kinda Victorian era in the complete lack of any hint of sex. And it also doesn't remark on the fact that Robin and Marian are married like 20 years and don't have any kids.
Cautions as to reading it? The dialogue goes between implausible and ridiculous, with Mr. Gilbert trying out a lot of Fake Medievalism that can only be enjoyed as camp, but he also often breaks away from such things for lovely little descriptive passages about the woods or a village or something, so I'd not rate the writing as bad overall. And these are stock characters. We get some shading, such as Robin having a temper he needs to control, or the sheriff not seeming like a particularly bad guy, just a greedy one who wants things quiet so he can get rich. Maid Marian doesn't have a huge presence in the book, though she does run off to the forest to find Robin, hiding out and spying on him, so she displays a little big of agency herself.