What your grammar book says is mostly correct, e.g. клева́ть (to peck) is conjugated клюю́, клюёшь, клюёт; воева́ть (to be at war) - вою́ю, вою́ешь, вою́ет.
However, when what is left after removing -е́ва ends in ж, ш, щ, ц, or ч, then -у is added instead of -ю so
e.g. линчева́ть (to lynch) becomes линчу́ю, линчу́ешь, линчу́ет. This is more of a rule than an exception as it can be seen throughout the Russian grammar. For instance, there is the suffix -ёнок meaning 'baby', 'cub': тигрёнок ('baby tiger'), лисёнок ('baby fox'), etc. After the above consonants it becomes -онок: мышо́нок, галчо́нок. These consonants are called 'unpaired with regard to hardness/softness' (непа́рные по твёрдости-мя́гкости): ч and щ are always soft, while ж, ш and ц are always hard. Thus, the choice of the following vowel (у or ю, о or е) is determined by orthography rather than phonetics. The orthographical tradition prescribes using у (not ю) after these consonants. In fact, it is one of the first rules Russian first-graders learn:
ЧУ-ЩУ пиши́ с бу́квой У.