I have two problems with this: first, it suggests that the reader is seeking something which directly lifts the spirits, by infusing them with positive sentiment, rather than lifting their spirits more obliquely by being a really good book. PG Wodehouse lifts the sprits, because he is clever and funny, but nobody would call his books “heartwarming”. Even, to take an extreme example, Jude The Obscure lifts the spirits, by moving you with its greatness as a work of literature, although admittedly you do have to get through about 48 hours of wanting to throw yourself out the window before the positive feelings kick in. In fact, is there anything of any artistic merit that could accurately be described as “heartwarming”? “Hamlet - a heartwarming tale of the ups and downs of a Danish prince”? “The Godfather - a heartwarming film about a close-knit Italian-American family”? I suggest not.
My second issue with “heartwarming” is it presumes my emotional response. I will decide how to react and at what temperature I want my bodily parts, thank you very much! For the same reason, I think it is a mistake to describe books as “hilarious” or “hysterically funny”. It seems to me both presumptuous of your audience and likely to be untrue for many, however good the work is. I’ve stuck with “humorous” for description of my book, since that describes the style; the reader can decide how funny (if at all) they think it is for themselves.
There is a marvellous slang idiom in Japanese used by teenagers - “Zenbei ga naita” - which means literally “All America wept”. They use this to describe things that are of no worth, because many cheesy Hollywood films are marketed in Japan with that hyperbolic slogan, so if you hear it you know the film isn’t worth seeing. The campaign for “heartwarming” to acquire a similar ironic meaning starts (and probably ends) here! Meanwhile, knowing that such irony is alive and well among the spiky-haired youth of Tokyo is enough to raise the temperature of your internal organs.