You are going to Greece at the perfect time -- everything's "open" nothing's crowded, the rains have ceased but everything is still Green. What a joy to hear from another "Ancient History Nerd"!! You already have touched on 2 of the most useful ... Rick's is good for those with no background at all, who want a efficient way to approach an ancient site D-I-Y, and Rough Guide has the history for those who want to dig deeper. Nice thing about RG is that you don't have to carry the big heavy book -- it's set up so each section is separate: carefully pull it off the backing, put ONE staple midway between top/bottom, then use duct tape to create a spine. Presto! Instant pocket-size guide-ettes! In yr 1st Greece trip you'll want sections on Pelops, Crete, Athens & Around, Greek History & Archeology (succinct AND excellent), Food & Drink, Music & Culture AND "Greek" (basics and good short phrase-list). So useful & totable... you'll thank me!
Since u have the gift of time, I suggest going to the library & checking out some things for winter reading & musing: The BLUE GUIDE to GREECE (Mainland) and the BLUE GUIDE to CRETE. These are super-good on ancient sites & also good on early-modern (the long struggle against the turks). CADOGAN GUIDE to CRETE and CADOGAN GUIDE to PELOPONNESE & Athens --altho not updated for some years -- are a joy simply to read, supple prose by writer Dana Facaros. Note: background can make SO much difference in how a site resonates w. you. Beside the ruins of Castle Frangokastello in Crete are busts with Greek-only legends. Only from Cadogan Guide did I know of the heroism and cruel deaths of Daskaloyannis & Dalianiis rising up against Turks in 18th & 19th C. Also, on my 1st Crete visit, we took the "Old Road" inland from big north highway... stopping for herds of goats, going thru tiny villages (I highly recommend this). In one town at high noon, we stopped in a silent, unpeopled, town center. In an island there was a cenotaph with carving of figures surrounded by flames. Next to it was a gold metal wreath. We felt something awful was being commemorated. Only later did I learn of the fate of Cretan villages who resisted the Nazis. In this one, ALL the men were executed & most of the buildings torched.
If you enjoy authentic history conveyed thru fiction you will devour the Mary Renault novels (they're good enough that, after reading her trilogy on Alexander, I scored 100% on tests in a U of Penn Alexander-era course without even buying the textbook). Some titles & topics: "The King Must Die" (Crete- the Theseus myths), "Last of the Wine" (Athens during key Golden=Age period at start of Pelop. War), "The Praise Singer" (early poets in post-HOmer era, just pre-democracy in the city-states). Spoiler alert: in appendix to each she gives historic materials/personages she based each book on. These all available in paperback in new/used bookstores & online.
Here are my 2 favorite online Maps for planning in your 2 areas: click & each will get huuuuge; shows not only roads, also terrain -- http://euro-map.com/karty-grecii/peloponnes/podrobnaya-turisticheskaya-karta-peloponnesa.jpg and http://www.explorecrete.com/crete-maps/images/Crete-eot.png Roads in Pelops are not totally updated. Nearer the date, if you use http://www.viamichelin.com/web/Routes you can see best routes for your interests. PELOPONNESE - the Argolid is the most landmark-dense area & Nafplio is ideal base for exploring. It used to be that counter-clockwise around the rim was best... but with road upgrades, one can go to Tripoli then S.W. thru mountains to Olympia, then down coast to Pylos, (I can suggest super place/s to stay) then hit Messine then go East.. stopping at Mystras/Monemvasia if medievalism floats your boat. CRETE -- Frankly Eastern Crete doesn't lure me much; in 6 trips have not gotten beyond Palace of Malia. Western & S. have SO much not only ancient but dramatic landscapes. Happy planning!