The description of the arcane grimoire magic item states (TCE, p. 120; emphasis mine):
While you are holding this leather-bound book, you can use it as a spellcasting focus for your wizard spells, and you gain a bonus to spell attack rolls and to the saving throw DCs of your wizard spells. The bonus is determined by the book’s rarity.
I do not understand the purpose of the word "and" after the second comma. Hence why I still have this question in my head:
Do I need to use the arcane grimoire as a spellcasting focus to gain its bonuses?
If a wizard does not need to use it as a spellcasting focus to gain its bonuses, they could very well "stack it" with an imbued wood focus (ERLW, p. 277; WGE, p. 114):
An imbued wood focus is a rod, staff, or wand cut from a tree infused with extraplanar energy. If you're a spellcaster, you can use this as a spellcasting focus.
When you cast a damage-dealing spell using this item as your spellcasting focus, you gain a +1 bonus to one damage roll of the spell, provided the damage is of the type associated with the item's wood. The types of wood and their associated damage types are listed in the Imbued Wood Focus table.