Print sources
There are lots of Forgotten Realms materials regarding deities, published over almost all editions of the game. For example, a lot of details are available in AD&D 2e sourcebook Faiths and Avatars (F&A), a 192-page tome dedicated only to the Faerunian pantheon. A tome of similar length is Faiths and Pantheons (F&P), this time for the 3e of the game. It is worth mentioning that the two books have somewhat different strengths though: F&A spends more space on clergy (day-to-day activities, holy days, ceremonies, priestly vestments - a color picture is given, adventuring garb), while F&P gives more details about the past mythology of the deities and their relationships with each other.
If you want to stick to 5e material, of course you also have the Sword Coast Adventurer's Guide, but the information will be relatively limited as deities are covered only as a chapter.
Novels
Lathander is a deity that is mentioned/featured in many FR novels. If you want to read about how their clergy or associated orders behave, you can try to have a look at this old list from candlekeep. (For example, it mentions that The Year of Rogue Dragon trilogy features Pavel Shemov, a cleric of Lathander.)
As mentioned in a comment by @Baergren, a more recent novel is The Reaver from the Sundering series, which discusses the "reemerging" of Lathander after a century-old disappearance.
Online sources
Regarding the Lathander/Amaunator issue, you can have a look at this post.
You can also read some information in the Forgotten Realms wikia.
Finally, another excellent website for finding all kinds of FR-related information is Candlekeep.com. It even gets posts from Ed Greenwood, the creator of the setting. If you use your favorite web search engine with the string "site:candlekeep.com lathander", you will find plenty of interesting things.
According to Volo's Guide to Monsters (p.33), Mordenkainen's Tome of Foes (p.29) and Out of the Abyss (p.246), Gnolls are created in the wake of Yeenoghu's great rampages across the planes. Originally, they were packs of hyenas that feasted on the corpses left behind by the great Demon Lord. They now breed true, but it's entirely possible that new tribes of Gnolls could spawn the next time Yeenoghu goes on rampage. It is reasonable to believe that this demonic origin has granted Gnolls the Abyssal language.
However, Volo's Guide to Monsters (p.35), also mentions in the "Language" section :
When gnoll leaders must share complex concepts with each other, they use a broken form of Abyssal gifted to them by Yeenoghu. The gnoll language lacks a script or written form, though elite gnolls can use their limited knowledge of Abyssal to leave messages.
This leads me to believe that the Gnoll language is not a dialect of Abyssal, but rather a very primitive collection of grunts and howls, and that only Gnoll leaders and probably also Gnoll cultists of Yeenoghu can actually speak Abyssal, albeit in a broken form.
To further prove that point, Volo's Guide to Monsters (p.153-155) presents four new types of Gnolls : the Flind, the Flesh Gnawer, the Hunter and the Witherling. Of those, only the Flind has the Abyssal language in its stat block, and it is also the only one to be described as a pack leader.
Best Answer
I can not say if there is any canon about the specific combination of gnoll and cleric of Erythnul. However, a Pathfinder third party publisher has a large page regarding gnolls, where they state the following (emphasis mine):
Additionally, if we look at the wiki on Erythnul it states that (emphasis mine):
Combining the two, I can imagine 2 scenes (depending on what is available):
Scenario 1 (No prisoners):
Scenario 2 (Prisoners were taken at whatever incident killed your party member):
PS: I never realized how... 'savage' Gnolls can be before trying to answer this question.