Start Siliguri up Old Cart Road but after a few kms past Sukna, before Rangtong there is a right turn by a small stretch of restaurants/hotels and shops.
This heads into very quiet, beautiful, misty tea gardens. Going North then East up sometimes very steep (the first time in a years cycling that I had to get off my bike and push! on tarmac) slopes. The effort is worth it, its so quiet and great views. Half up and half down, one valley Nepali and the next Hindu.
Tarmac all the way, but sometimes patchy; Only small shops on the way, some small villages (no veg, tea or food available), mostly green lush landscapes and peace and quiet. 1.5 - 2 days ride to main road.
Then 31A main road, over Teesta to Melli.
Follow river Rangeet road (Melli-Nayarbazar road) following river valley to Jorethang. This road is appalling due to many landslides along the way the road is mostly mud, with some sealed road. Not steep as it follows river so is not too bad!
Jorethang: town with many shops, market, hotels, restaurants
Same road conditions to Soreng going West then North. Turning from river road to Soreng very good road, sealed.
Soreng to Kaluk: Steady climb, just after Soreng there's about 2 kms of poor compacted rock road but then turns to sealed road.
Kaluk: small town with chai and snack foods, hotel. Nice little place.
Kaluk - Pelling; I as told not to take the road going East to Legship so I went the West road to Pelling: half-half steady down then up, all sealed.
Pelling: Touristy big town: Hotels, internet, ATM, restaurants> Ladakh guest house at top of town with Dorms. for 150 Rs. Good, clean, give you hot water for shower in a bucket. Go to Hotel Kabol at top for very good Thuckpa, Tibetan hand-made noodle soup.
Pelling - Yuksom: majority down. Good sealed for most. Yuksom: small town many hotels, Hotel D....big place on left at top 100 Rs. dorm. Nice place, quiet this time of year
Yuksom - Ravangla via Legship: Poor roads, compacted rock. Pass through Tashiding: small town, big monastery, hotels and food here. Climb from Legship. Legship small town with food
Ravangla - Yangang and down to river: downhill, mostly good sealed road except one big landslide that tore away road, still navigable. Both Ravangla and Yangang hotels and food.
Main road to Gangtok. (no travel agent in Gangtok would issue me with a permit for Gurudongmar lake, not available to foreigners anymore)
Gangtok - Mangan: up to get out of Gangtok then down down down. Poor road, broken and 3 landslides along the way. First one the army were installing a bridge to replace road that disappeared. All three passable if carry bike!
Mangan - Chungtung. No road but all mud and compacted rock, all under repair, hard going, some pushing.
But just before Chungtung police check point: need permit beyond this point, get it at Mangan: District Collector, DAC, North Sikkim, Mangan. I didn't go further North as did not have permit and road supposed to be very bad indeed.
Mangan - Singtam: South of Mangan is small road that follows Teesta river via Dikchu: patchy road, some tarmac some compacted rock. Dikchu (On Google map it only shows Dikchu as a river but at convergence of Teest and Dikchu is a town) has food and hotels.
Singtam-Namchi via Pabong and Phongla: Up all way to Phongla then down to Namchi, good road. small shops and villages but no food or hotels. Phongla to Namchi good road but patches of landslide repair.
Namchi back toward Phongla to the Bhanjang road to Melli: Down, mostly good but patchy. Before Melli road divides, I went right and road turns to downhill steep track, rock and gravel, v steep!. Do not attempt going up, you will be pushing all the way.
Teesta town/bazar - Ghum via Jorbunblow on road 12: Up all the way, tough, crazy steepness, but forests and tea plantations, again got off and pushed sometimes. Poor roads, mini potholed, bumpy. Villages with snacks, tea and food on way, no hotels.
And then your book has the rest. Stopped in Mirik, best place to stay Buddha Lodge, very clean, cheapest (450 Rs.!!!)in Krishnagar, Mirik.