Ttyjfyk

I faintly remember of a D&D 3.5 book, but I can not really remember its title. The book had a lovecraftian flavor, it was official and it was about old gods and monsters, that existed in the background, and the DM could create whole campaigns about them.

It usually proposed events to happen in the campaign, and the players had to figure out ways to divert the coming/awakening off the big bad guys.

Does anybody happen to know the title of the book?

You're probably thinking of Elder Evils (Dec. 2007) that contains several cosmic big bads (although not all can be described as Lovecraftian), the big bads' minions, and their plots that see them trying to alter or destroy the worlds. It includes rules for, for example, signs of the coming apocalypses that telegraph their arrival.

For example, the book summarizes chapter 2 about Atropus as follows:

This moonlike orb is the stillborn afterbirth of the world’s creation, an undead entity that desires nothing less than the end of the entire multiverse. (4)

And, for example, the book summarizes chapter 7 about Ragnorra as follows:

Arising in a time before creation, Ragnorra is a force of perverted life that births monstrosities from her swollen body. The deities cast her into the sky, but she reappears as a blood-red comet every 500 years. Dreadful seed and falling stars produce monstrous offspring in worlds Ragnorra approaches—and now she smashes into worlds to remake them directly. (ibid.)

Unlike the similarly titled Exemplars of Evil (Sept. 2007), there's little for players in Elder Evils so it's not often mentioned in discussions of, for instance, character-building.

The book you are looking for is probably Elder Evils, published December of 2007 by WotC and written by Robert J. Schwalb.

This book is WYSOTT (what you see on the tin), so is the most likely candidate. It covers apocalypse settings and campaigns and those who serve the various evils presented some of which are decidedly Lovecraftian.

Other sources for related material include Exemplars of Evil, Fiendish Codex I & II, Fiend Folio, Tome of Magic (the binder class, which oddly copied verbatim from an out of copyright book in real life for almost all of the material), Champions of Ruin, Heroes of Horror, Libris Mortis, Lords of Madness, Complete Divine, Planar Handbook, Manual of the Planes, the PHB, the DMG, and the infamous Book of Vile Darkness and equally infamous Book of Exalted Deeds (which ironically has a fair amount about evil instead of good).