I purchased three guide books, probably one too many:
the lonely planet guide to Nepal, which covers activities other than trekking,
the Mountaineer’s book Trekking Nepal 8th edition by Stephen Bezruchka & Alonzo Lyons. Bezruchka is a physician who has spent years trekking in Nepal, so the Mountaineer’s book is especially strong on medical advice; but while there is good general information on the routes, they don’t review individual tea houses. and
the lonely planet e-book Nepal: Trekking, which was cheap and seems to be an abridged version of the lonely planet Trekking in the Nepal Himalayas, which is the one to get if you want detailed reviews of individual tea houses along the trek.