Symbol and Image in Celtic Religious Art and The Gods of the Celts
It's difficult not to lump these two books together in one way or another because in many ways they're pretty much the same book, just done in a slightly different way.
Of the two, The Gods of the Celts is the older and - I found - slightly less readable of the two. Partly it's a difference in formatting that makes Symbol and Image a better read; but mostly I think it's to do with the fact that Symbol and Image is more up to date in terms of information, and as a (only slightly, admittedly) more recent work Green seems to have developed her writing style which makes for a more engaging read overall. It has to be said that the illustrations in Symbol and Image also help to put what she's saying into a better context. What can I say? I like the pretty pictures...
For the most part Green covers Gaulish examples, and to a lesser extent she then covers the British and Gaelic evidence. This is usual - Green's area of expertise certainly lies in Gaul so that creates a natural bias, I suppose. When she covers things outside of this area she often relies on the work of Anne Ross, which is some cause for concern and caution. Much of Ross' work is valuable but now dated and Green doesn't make as much effort to balance out Ross' opinions with work from different authors who approach the subject from a different academic viewpoint as she does when she's more confident in her subject. First and foremost, though, Green is an archaeologist and not a Celticist as such, and it's important to remember this as you read.
Another problem with Green's work (in general) is her insistence on pigeon-holing deities into specific roles - sky-gods, sun-gods, chthonic deities and so on. This is a very Classical interpretation, which isn't necessarily appropriate in a Celtic context and it's a shame she doesn't really make much effort to consider or explore a more native view. The insistence on sun-gods is something that grates - what makes them sun-gods? Why the disparity between her interpretation and the distinct lack of any overt connections with the sun, or a solar role, in the surviving myths? Some exploration of this would have been nice, because otherwise it seems she's just regurgitating a commonly held view with little thought to whether it's actually correct or not. Neither does she really explain why she sees wheel-shaped iconography associated with some gods as being 'sun-wheels', except (as far as I could see) to reference an article written by herself. A little exploration of this would have been nice, to show some balance at least.
Having said all that, the books are actually quite good. One of Green's advantages is that she's able to write simply and clearly without losing her audience, and she does a good job of giving a good overview of the subject whilst also raising some points to ponder. Sometimes she delves a little too deeply into using jargon and technical terms without really explaining them (or maybe she just assumes the reader has a larger vocabulary than publishers tend to these days), but this was more of a problem with The Gods of the Celts than it was with Symbol and Image. To be fair, it's nothing a dictionary or a quick google can help to solve, though it could be annoying.
Symbol and Image focuses more on the deities and what their associated iconography and symbolism implies about their roles (so it kinda does what it says on the tin...). Each chapter focuses on a particular subject - the female image, the male image, triplism, the divine marriage, symbolism in the natural world, and what the style of the iconography itself implies. Within each chapter Green focuses on particular subheadings that are relevant - horned gods, iconography of birds, warrior cults and so on, as well as particular deities and divine couples.
The Gods of the Celts covers a lot of the same stuff, laid out in a different way. Chapters cover water gods and healing, war, death and the underworld, animals and animism, 'cults of sun and sky' and so on. The last example involves some gritting of teeth in particular, but in general these bits are easily read around if you find yourself disagreeing with it like I did. While there's a big overlap with Symbol and Image, The Gods of the Celts does a better job of giving a more rounded idea of Celtic religion and expression as a whole. Green goes into more detail about how the gods were worshipped and so it's a better read in that respect, because it probably caters more to what any budding Recon wants to know.
Because they're so similar, sometimes it seemed like Symbol and Image is a reworking of the first book but with a few more bits and pieces to add to the discussion. Each book has something unique to offer though, and they're both worth a read - they're not the be all and end all of the subject, but they're a good overview and Green gives good references and bibliographies to help you delve a little deeper if you want to. Since I found Symbol and Image to be a more engaging read, and better formatted, I'd suggest starting with that one if you're looking for a good book about the gods. The Gods of the Celts will do the job as well, and if you're looking to get a more rounded view as a beginner (or a refresher, or wider context, as a more advance type) - an idea of the religion and the gods - then start with The Gods of the Celts.
They're both good books to expand your general knowledge about Celtic religion, but bear in mind the continental and Romano-Celtic (encompassing Roman Britain as well, then) bias. You'll probably be disappointed if you're looking for a good indepth study of the Irish gods, for example, but even so they're worth a read if you want to get an idea of the different nuances in religious belief and expression across the various Celtic cultures.